diff --git a/PHPCSUtils/BackCompat/BCFile.php b/PHPCSUtils/BackCompat/BCFile.php
index 121f8b24..9c761144 100644
--- a/PHPCSUtils/BackCompat/BCFile.php
+++ b/PHPCSUtils/BackCompat/BCFile.php
@@ -141,6 +141,8 @@ public static function getDeclarationName(File $phpcsFile, $stackPtr)
      * ```php
      *   'property_visibility' => string,  // The property visibility as declared.
      *   'visibility_token'    => integer, // The stack pointer to the visibility modifier token.
+     *   'property_readonly'   => bool,    // TRUE if the readonly keyword was found.
+     *   'readonly_token'      => integer, // The stack pointer to the readonly modifier token.
      * ```
      *
      * PHPCS cross-version compatible version of the `File::getMethodParameters()` method.
@@ -217,6 +219,7 @@ public static function getMethodParameters(File $phpcsFile, $stackPtr)
         $typeHintEndToken = false;
         $nullableType     = false;
         $visibilityToken  = null;
+        $readonlyToken    = null;
 
         for ($i = $paramStart; $i <= $closer; $i++) {
             // Check to see if this token has a parenthesis or bracket opener. If it does
@@ -345,6 +348,11 @@ public static function getMethodParameters(File $phpcsFile, $stackPtr)
                         $visibilityToken = $i;
                     }
                     break;
+                case T_READONLY:
+                    if ($defaultStart === null) {
+                        $readonlyToken = $i;
+                    }
+                    break;
                 case T_CLOSE_PARENTHESIS:
                 case T_COMMA:
                     // If it's null, then there must be no parameters for this
@@ -377,6 +385,12 @@ public static function getMethodParameters(File $phpcsFile, $stackPtr)
                     if ($visibilityToken !== null) {
                         $vars[$paramCount]['property_visibility'] = $tokens[$visibilityToken]['content'];
                         $vars[$paramCount]['visibility_token']    = $visibilityToken;
+                        $vars[$paramCount]['property_readonly']   = false;
+                    }
+
+                    if ($readonlyToken !== null) {
+                        $vars[$paramCount]['property_readonly'] = true;
+                        $vars[$paramCount]['readonly_token']    = $readonlyToken;
                     }
 
                     if ($tokens[$i]['code'] === T_COMMA) {
@@ -400,6 +414,7 @@ public static function getMethodParameters(File $phpcsFile, $stackPtr)
                     $typeHintEndToken = false;
                     $nullableType     = false;
                     $visibilityToken  = null;
+                    $readonlyToken    = null;
 
                     ++$paramCount;
                     break;
diff --git a/PHPCSUtils/Tokens/Collections.php b/PHPCSUtils/Tokens/Collections.php
index 1af6e74f..3b221cf4 100644
--- a/PHPCSUtils/Tokens/Collections.php
+++ b/PHPCSUtils/Tokens/Collections.php
@@ -503,6 +503,7 @@ class Collections
      * DEPRECATED: Modifier keywords which can be used for a property declaration.
      *
      * @since 1.0.0-alpha1
+     * @since 1.0.0-alpha4 Added the T_READONLY token for PHP 8.1 readonly properties.
      *
      * @deprecated 1.0.0-alpha4 Use the {@see Collections::propertyModifierKeywords()} method instead.
      *
@@ -514,6 +515,7 @@ class Collections
         \T_PROTECTED => \T_PROTECTED,
         \T_STATIC    => \T_STATIC,
         \T_VAR       => \T_VAR,
+        \T_READONLY  => \T_READONLY,
     ];
 
     /**
diff --git a/PHPCSUtils/Utils/FunctionDeclarations.php b/PHPCSUtils/Utils/FunctionDeclarations.php
index f636b712..7ad7d1df 100644
--- a/PHPCSUtils/Utils/FunctionDeclarations.php
+++ b/PHPCSUtils/Utils/FunctionDeclarations.php
@@ -343,6 +343,8 @@ public static function getProperties(File $phpcsFile, $stackPtr)
      * ```php
      *   'property_visibility' => string,  // The property visibility as declared.
      *   'visibility_token'    => integer, // The stack pointer to the visibility modifier token.
+     *   'property_readonly'   => bool,    // TRUE if the readonly keyword was found.
+     *   'readonly_token'      => integer, // The stack pointer to the readonly modifier token.
      * ```
      *
      * Main differences with the PHPCS version:
@@ -361,6 +363,7 @@ public static function getProperties(File $phpcsFile, $stackPtr)
      * @since 1.0.0-alpha4 Added support for PHP 8.0 constructor property promotion.
      * @since 1.0.0-alpha4 Added support for PHP 8.0 identifier name tokenization.
      * @since 1.0.0-alpha4 Added support for PHP 8.0 parameter attributes.
+     * @since 1.0.0-alpha4 Added support for PHP 8.1 readonly keyword for constructor property promotion.
      *
      * @param \PHP_CodeSniffer\Files\File $phpcsFile The file being scanned.
      * @param int                         $stackPtr  The position in the stack of the function token
@@ -426,6 +429,7 @@ public static function getParameters(File $phpcsFile, $stackPtr)
         $typeHintEndToken = false;
         $nullableType     = false;
         $visibilityToken  = null;
+        $readonlyToken    = null;
 
         for ($i = $paramStart; $i <= $closer; $i++) {
             if (isset(Collections::parameterTypeTokens()[$tokens[$i]['code']]) === true
@@ -475,6 +479,10 @@ public static function getParameters(File $phpcsFile, $stackPtr)
                     $visibilityToken = $i;
                     break;
 
+                case \T_READONLY:
+                    $readonlyToken = $i;
+                    break;
+
                 case \T_CLOSE_PARENTHESIS:
                 case \T_COMMA:
                     // If it's null, then there must be no parameters for this
@@ -511,6 +519,12 @@ public static function getParameters(File $phpcsFile, $stackPtr)
                     if ($visibilityToken !== null) {
                         $vars[$paramCount]['property_visibility'] = $tokens[$visibilityToken]['content'];
                         $vars[$paramCount]['visibility_token']    = $visibilityToken;
+                        $vars[$paramCount]['property_readonly']   = false;
+                    }
+
+                    if ($readonlyToken !== null) {
+                        $vars[$paramCount]['property_readonly'] = true;
+                        $vars[$paramCount]['readonly_token']    = $readonlyToken;
                     }
 
                     if ($tokens[$i]['code'] === \T_COMMA) {
@@ -534,6 +548,7 @@ public static function getParameters(File $phpcsFile, $stackPtr)
                     $typeHintEndToken = false;
                     $nullableType     = false;
                     $visibilityToken  = null;
+                    $readonlyToken    = null;
 
                     ++$paramCount;
                     break;
diff --git a/PHPCSUtils/Utils/Variables.php b/PHPCSUtils/Utils/Variables.php
index 027e5e73..14c92e7c 100644
--- a/PHPCSUtils/Utils/Variables.php
+++ b/PHPCSUtils/Utils/Variables.php
@@ -89,6 +89,7 @@ class Variables
      * @since 1.0.0
      * @since 1.0.0-alpha4 Added support for PHP 8.0 union types.
      * @since 1.0.0-alpha4 No longer gets confused by PHP 8.0 property attributes.
+     * @since 1.0.0-alpha4 Added support for PHP 8.1 readonly properties.
      *
      * @param \PHP_CodeSniffer\Files\File $phpcsFile The file being scanned.
      * @param int                         $stackPtr  The position in the stack of the `T_VARIABLE` token
@@ -101,6 +102,7 @@ class Variables
      *                 'scope'           => string,  // Public, private, or protected.
      *                 'scope_specified' => boolean, // TRUE if the scope was explicitly specified.
      *                 'is_static'       => boolean, // TRUE if the static keyword was found.
+     *                 'is_readonly'     => boolean, // TRUE if the readonly keyword was found.
      *                 'type'            => string,  // The type of the var (empty if no type specified).
      *                 'type_token'      => integer, // The stack pointer to the start of the type
      *                                               // or FALSE if there is no type.
@@ -133,6 +135,7 @@ public static function getMemberProperties(File $phpcsFile, $stackPtr)
         $scope          = 'public';
         $scopeSpecified = false;
         $isStatic       = false;
+        $isReadonly     = false;
 
         $startOfStatement = $phpcsFile->findPrevious(
             [
@@ -165,6 +168,9 @@ public static function getMemberProperties(File $phpcsFile, $stackPtr)
                 case \T_STATIC:
                     $isStatic = true;
                     break;
+                case \T_READONLY:
+                    $isReadonly = true;
+                    break;
             }
         }
 
@@ -205,6 +211,7 @@ public static function getMemberProperties(File $phpcsFile, $stackPtr)
             'scope'           => $scope,
             'scope_specified' => $scopeSpecified,
             'is_static'       => $isStatic,
+            'is_readonly'     => $isReadonly,
             'type'            => $type,
             'type_token'      => $typeToken,
             'type_end_token'  => $typeEndToken,
diff --git a/Tests/BackCompat/BCFile/GetMemberPropertiesTest.inc b/Tests/BackCompat/BCFile/GetMemberPropertiesTest.inc
index 8673049a..034ff308 100644
--- a/Tests/BackCompat/BCFile/GetMemberPropertiesTest.inc
+++ b/Tests/BackCompat/BCFile/GetMemberPropertiesTest.inc
@@ -242,6 +242,31 @@ $anon = class() {
     /* testPHP8DuplicateTypeInUnionWhitespaceAndComment */
     // Intentional fatal error - duplicate types are not allowed in union types, but that's not the concern of the method.
     public int |string| /*comment*/ INT $duplicateTypeInUnion;
+
+    /* testPHP81Readonly */
+    public readonly int $readonly;
+
+    /* testPHP81ReadonlyWithNullableType */
+    public readonly ?array $readonlyWithNullableType;
+
+    /* testPHP81ReadonlyWithUnionType */
+    public readonly string|int $readonlyWithUnionType;
+
+    /* testPHP81ReadonlyWithUnionTypeWithNull */
+    protected ReadOnly string|null $readonlyWithUnionTypeWithNull;
+
+    /* testPHP81OnlyReadonlyWithUnionType */
+    readonly string|int $onlyReadonly;
+
+    /* testPHP81OnlyReadonlyWithUnionTypeMultiple */
+    readonly \InterfaceA|\Sub\InterfaceB|false
+        $onlyReadonly;
+
+    /* testPHP81ReadonlyAndStatic */
+    readonly private static ?string $readonlyAndStatic;
+
+    /* testPHP81ReadonlyMixedCase */
+    public ReadONLY static $readonlyMixedCase;
 };
 
 $anon = class {
diff --git a/Tests/BackCompat/BCFile/GetMemberPropertiesTest.php b/Tests/BackCompat/BCFile/GetMemberPropertiesTest.php
index 766a83aa..574248c1 100644
--- a/Tests/BackCompat/BCFile/GetMemberPropertiesTest.php
+++ b/Tests/BackCompat/BCFile/GetMemberPropertiesTest.php
@@ -70,9 +70,6 @@ public function testGetMemberProperties($identifier, $expected)
             $expected['type_end_token'] += $variable;
         }
 
-        // Temporarily ignore the `is_readonly` key until support for readonly properties has been synced.
-        unset($result['is_readonly']);
-
         $this->assertSame($expected, $result);
     }
 
@@ -94,6 +91,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => false,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -106,6 +104,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => false,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => '?int',
                     'type_token'      => -2, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
@@ -118,6 +117,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -130,6 +130,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => 'string',
                     'type_token'      => -2, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
@@ -142,6 +143,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'protected',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -154,6 +156,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'protected',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => 'bool',
                     'type_token'      => -2, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
@@ -166,6 +169,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'private',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -178,6 +182,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'private',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => 'array',
                     'type_token'      => -2, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
@@ -190,6 +195,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => false,
                     'is_static'       => true,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -202,6 +208,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => false,
                     'is_static'       => true,
+                    'is_readonly'     => false,
                     'type'            => '?string',
                     'type_token'      => -2, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
@@ -214,6 +221,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => false,
                     'is_static'       => true,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -226,6 +234,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => false,
                     'is_static'       => true,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -238,6 +247,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => true,
                     'is_static'       => true,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -250,6 +260,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'protected',
                     'scope_specified' => true,
                     'is_static'       => true,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -262,6 +273,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'private',
                     'scope_specified' => true,
                     'is_static'       => true,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -274,6 +286,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => false,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -286,6 +299,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => true,
                     'is_static'       => true,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -298,6 +312,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'protected',
                     'scope_specified' => true,
                     'is_static'       => true,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -310,6 +325,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'private',
                     'scope_specified' => true,
                     'is_static'       => true,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -322,6 +338,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => 'float',
                     'type_token'      => -6, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -6, // Offset from the T_VARIABLE token.
@@ -334,6 +351,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => 'float',
                     'type_token'      => -13, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -13, // Offset from the T_VARIABLE token.
@@ -346,6 +364,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => true,
                     'is_static'       => true,
+                    'is_readonly'     => false,
                     'type'            => '?string',
                     'type_token'      => -6, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -6, // Offset from the T_VARIABLE token.
@@ -358,6 +377,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => true,
                     'is_static'       => true,
+                    'is_readonly'     => false,
                     'type'            => '?string',
                     'type_token'      => -17, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -17, // Offset from the T_VARIABLE token.
@@ -370,6 +390,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'protected',
                     'scope_specified' => true,
                     'is_static'       => true,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -382,6 +403,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'protected',
                     'scope_specified' => true,
                     'is_static'       => true,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -394,6 +416,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'protected',
                     'scope_specified' => true,
                     'is_static'       => true,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -406,6 +429,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'private',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -418,6 +442,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'private',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -430,6 +455,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'private',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -442,6 +468,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'private',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -454,6 +481,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'private',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -466,6 +494,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'private',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -478,6 +507,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'private',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -490,6 +520,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => '?array',
                     'type_token'      => -2, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
@@ -502,6 +533,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => '\MyNamespace\MyClass',
                     'type_token'      => ($php8Names === true) ? -2 : -5, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
@@ -514,6 +546,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'private',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => '?ClassName',
                     'type_token'      => -2, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
@@ -526,6 +559,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'protected',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => '?Folder\ClassName',
                     'type_token'      => ($php8Names === true) ? -2 : -4, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
@@ -538,6 +572,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => '\MyNamespace\MyClass\Foo',
                     'type_token'      => ($php8Names === true) ? -15 : -18, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
@@ -550,6 +585,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'private',
                     'scope_specified' => true,
                     'is_static'       => true,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -566,6 +602,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -578,6 +615,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => '',
                     'type_token'      => false,
                     'type_end_token'  => false,
@@ -590,6 +628,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => true,
                     'is_static'       => true,
+                    'is_readonly'     => false,
                     'type'            => 'miXed',
                     'type_token'      => -2, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
@@ -602,6 +641,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'private',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => '?mixed',
                     'type_token'      => -2, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
@@ -614,6 +654,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => '?namespace\Name',
                     'type_token'      => ($php8Names === true) ? -2 : -4, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
@@ -626,6 +667,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => 'int|float',
                     'type_token'      => -4, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
@@ -638,6 +680,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'private',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => 'MyClassA|\Package\MyClassB',
                     'type_token'      => ($php8Names === true) ? -4 : -7, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
@@ -650,6 +693,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'protected',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => 'array|bool|int|float|NULL|object|string',
                     'type_token'      => -14, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
@@ -662,6 +706,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => false,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => 'false|mixed|self|parent|iterable|Resource',
                     'type_token'      => -12, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
@@ -674,7 +719,9 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => true,
                     'is_static'       => false,
-                    'type'            => 'callable||void', // Missing static, but that's OK as not an allowed syntax.
+                    'is_readonly'     => false,
+                    // Missing static, but that's OK as not an allowed syntax.
+                    'type'            => 'callable||void',
                     'type_token'      => -6, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
                     'nullable_type'   => false,
@@ -686,6 +733,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => '?int|float',
                     'type_token'      => -4, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
@@ -698,6 +746,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => 'null',
                     'type_token'      => -2, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
@@ -710,6 +759,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => 'false',
                     'type_token'      => -2, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
@@ -722,6 +772,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => 'bool|FALSE',
                     'type_token'      => -4, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
@@ -734,6 +785,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => 'object|ClassName',
                     'type_token'      => -4, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
@@ -746,6 +798,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => 'iterable|array|Traversable',
                     'type_token'      => -6, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
@@ -758,18 +811,124 @@ public function dataGetMemberProperties()
                     'scope'           => 'public',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => 'int|string|INT',
                     'type_token'      => -10, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
                     'nullable_type'   => false,
                 ],
             ],
+            'php8.1-readonly-property' => [
+                '/* testPHP81Readonly */',
+                [
+                    'scope'           => 'public',
+                    'scope_specified' => true,
+                    'is_static'       => false,
+                    'is_readonly'     => true,
+                    'type'            => 'int',
+                    'type_token'      => -2, // Offset from the T_VARIABLE token.
+                    'type_end_token'  => -2, // Offset from the T_VARIABLE token.
+                    'nullable_type'   => false,
+                ],
+            ],
+            'php8.1-readonly-property-with-nullable-type' => [
+                '/* testPHP81ReadonlyWithNullableType */',
+                [
+                    'scope'           => 'public',
+                    'scope_specified' => true,
+                    'is_static'       => false,
+                    'is_readonly'     => true,
+                    'type'            => '?array',
+                    'type_token'      => -2, // Offset from the T_VARIABLE token.
+                    'type_end_token'  => -2, // Offset from the T_VARIABLE token.
+                    'nullable_type'   => true,
+                ],
+            ],
+            'php8.1-readonly-property-with-union-type' => [
+                '/* testPHP81ReadonlyWithUnionType */',
+                [
+                    'scope'           => 'public',
+                    'scope_specified' => true,
+                    'is_static'       => false,
+                    'is_readonly'     => true,
+                    'type'            => 'string|int',
+                    'type_token'      => -4, // Offset from the T_VARIABLE token.
+                    'type_end_token'  => -2, // Offset from the T_VARIABLE token.
+                    'nullable_type'   => false,
+                ],
+            ],
+            'php8.1-readonly-property-with-union-type-with-null' => [
+                '/* testPHP81ReadonlyWithUnionTypeWithNull */',
+                [
+                    'scope'           => 'protected',
+                    'scope_specified' => true,
+                    'is_static'       => false,
+                    'is_readonly'     => true,
+                    'type'            => 'string|null',
+                    'type_token'      => -4, // Offset from the T_VARIABLE token.
+                    'type_end_token'  => -2, // Offset from the T_VARIABLE token.
+                    'nullable_type'   => false,
+                ],
+            ],
+            'php8.1-readonly-property-with-union-type-no-visibility' => [
+                '/* testPHP81OnlyReadonlyWithUnionType */',
+                [
+                    'scope'           => 'public',
+                    'scope_specified' => false,
+                    'is_static'       => false,
+                    'is_readonly'     => true,
+                    'type'            => 'string|int',
+                    'type_token'      => -4, // Offset from the T_VARIABLE token.
+                    'type_end_token'  => -2, // Offset from the T_VARIABLE token.
+                    'nullable_type'   => false,
+                ],
+            ],
+            'php8.1-readonly-property-with-multi-union-type-no-visibility' => [
+                '/* testPHP81OnlyReadonlyWithUnionTypeMultiple */',
+                [
+                    'scope'           => 'public',
+                    'scope_specified' => false,
+                    'is_static'       => false,
+                    'is_readonly'     => true,
+                    'type'            => '\InterfaceA|\Sub\InterfaceB|false',
+                    'type_token'      => ($php8Names === true) ? -7 : -11, // Offset from the T_VARIABLE token.
+                    'type_end_token'  => -3, // Offset from the T_VARIABLE token.
+                    'nullable_type'   => false,
+                ],
+            ],
+            'php8.1-readonly-and-static-property' => [
+                '/* testPHP81ReadonlyAndStatic */',
+                [
+                    'scope'           => 'private',
+                    'scope_specified' => true,
+                    'is_static'       => true,
+                    'is_readonly'     => true,
+                    'type'            => '?string',
+                    'type_token'      => -2, // Offset from the T_VARIABLE token.
+                    'type_end_token'  => -2, // Offset from the T_VARIABLE token.
+                    'nullable_type'   => true,
+                ],
+            ],
+            'php8.1-readonly-mixed-case-keyword' => [
+                '/* testPHP81ReadonlyMixedCase */',
+                [
+                    'scope'           => 'public',
+                    'scope_specified' => true,
+                    'is_static'       => true,
+                    'is_readonly'     => true,
+                    'type'            => '',
+                    'type_token'      => false,
+                    'type_end_token'  => false,
+                    'nullable_type'   => false,
+                ],
+            ],
             'php8-property-with-single-attribute' => [
                 '/* testPHP8PropertySingleAttribute */',
                 [
                     'scope'           => 'public',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => 'string',
                     'type_token'      => -2, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
@@ -782,6 +941,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'protected',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => '?int|float',
                     'type_token'      => -4, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
@@ -794,6 +954,7 @@ public function dataGetMemberProperties()
                     'scope'           => 'private',
                     'scope_specified' => true,
                     'is_static'       => false,
+                    'is_readonly'     => false,
                     'type'            => 'mixed',
                     'type_token'      => -2, // Offset from the T_VARIABLE token.
                     'type_end_token'  => -2, // Offset from the T_VARIABLE token.
diff --git a/Tests/BackCompat/BCFile/GetMethodParametersTest.inc b/Tests/BackCompat/BCFile/GetMethodParametersTest.inc
index 91e57710..e5918b40 100644
--- a/Tests/BackCompat/BCFile/GetMethodParametersTest.inc
+++ b/Tests/BackCompat/BCFile/GetMethodParametersTest.inc
@@ -201,6 +201,11 @@ class ConstructorPropertyPromotionAndNormalParams {
     public function __construct(public int $promotedProp, ?int $normalArg) {}
 }
 
+class ConstructorPropertyPromotionWithReadOnly {
+    /* testPHP81ConstructorPropertyPromotionWithReadOnly */
+    public function __construct(public readonly ?int $promotedProp, ReadOnly private string|bool &$promotedToo) {}
+}
+
 /* testPHP8ConstructorPropertyPromotionGlobalFunction */
 // Intentional fatal error. Property promotion not allowed in non-constructor, but that's not the concern of this method.
 function globalFunction(private $x) {}
diff --git a/Tests/BackCompat/BCFile/GetMethodParametersTest.php b/Tests/BackCompat/BCFile/GetMethodParametersTest.php
index 217a2ea4..021e2a88 100644
--- a/Tests/BackCompat/BCFile/GetMethodParametersTest.php
+++ b/Tests/BackCompat/BCFile/GetMethodParametersTest.php
@@ -1748,6 +1748,7 @@ public function testPHP8ConstructorPropertyPromotionNoTypes()
             'nullable_type'       => false,
             'property_visibility' => 'public',
             'visibility_token'    => 6, // Offset from the T_FUNCTION token.
+            'property_readonly'   => false,
             'comma_token'         => 13,
         ];
         $expected[1] = [
@@ -1768,6 +1769,7 @@ public function testPHP8ConstructorPropertyPromotionNoTypes()
             'nullable_type'       => false,
             'property_visibility' => 'protected',
             'visibility_token'    => 16, // Offset from the T_FUNCTION token.
+            'property_readonly'   => false,
             'comma_token'         => 23,
         ];
         $expected[2] = [
@@ -1788,6 +1790,7 @@ public function testPHP8ConstructorPropertyPromotionNoTypes()
             'nullable_type'       => false,
             'property_visibility' => 'private',
             'visibility_token'    => 26, // Offset from the T_FUNCTION token.
+            'property_readonly'   => false,
             'comma_token'         => 33,
         ];
 
@@ -1817,6 +1820,7 @@ public function testPHP8ConstructorPropertyPromotionWithTypes()
             'nullable_type'       => false,
             'property_visibility' => 'protected',
             'visibility_token'    => 4, // Offset from the T_FUNCTION token.
+            'property_readonly'   => false,
             'comma_token'         => 11,
         ];
         $expected[1] = [
@@ -1837,6 +1841,7 @@ public function testPHP8ConstructorPropertyPromotionWithTypes()
             'nullable_type'       => true,
             'property_visibility' => 'public',
             'visibility_token'    => 13, // Offset from the T_FUNCTION token.
+            'property_readonly'   => false,
             'comma_token'         => 24,
         ];
         $expected[2] = [
@@ -1854,6 +1859,7 @@ public function testPHP8ConstructorPropertyPromotionWithTypes()
             'nullable_type'       => false,
             'property_visibility' => 'private',
             'visibility_token'    => 26, // Offset from the T_FUNCTION token.
+            'property_readonly'   => false,
             'comma_token'         => false,
         ];
 
@@ -1883,6 +1889,7 @@ public function testPHP8ConstructorPropertyPromotionAndNormalParam()
             'nullable_type'       => false,
             'property_visibility' => 'public',
             'visibility_token'    => 4, // Offset from the T_FUNCTION token.
+            'property_readonly'   => false,
             'comma_token'         => 9,
         ];
         $expected[1] = [
@@ -1904,6 +1911,56 @@ public function testPHP8ConstructorPropertyPromotionAndNormalParam()
         $this->getMethodParametersTestHelper('/* ' . __FUNCTION__ . ' */', $expected);
     }
 
+    /**
+     * Verify recognition of PHP8 constructor with property promotion using PHP 8.1 readonly keyword.
+     *
+     * @return void
+     */
+    public function testPHP81ConstructorPropertyPromotionWithReadOnly()
+    {
+        $expected    = [];
+        $expected[0] = [
+            'token'               => 11, // Offset from the T_FUNCTION token.
+            'name'                => '$promotedProp',
+            'content'             => 'public readonly ?int $promotedProp',
+            'has_attributes'      => false,
+            'pass_by_reference'   => false,
+            'reference_token'     => false,
+            'variable_length'     => false,
+            'variadic_token'      => false,
+            'type_hint'           => '?int',
+            'type_hint_token'     => 9, // Offset from the T_FUNCTION token.
+            'type_hint_end_token' => 9, // Offset from the T_FUNCTION token.
+            'nullable_type'       => true,
+            'property_visibility' => 'public',
+            'visibility_token'    => 4, // Offset from the T_FUNCTION token.
+            'property_readonly'   => true,
+            'readonly_token'      => 6, // Offset from the T_FUNCTION token.
+            'comma_token'         => 12,
+        ];
+        $expected[1] = [
+            'token'               => 23, // Offset from the T_FUNCTION token.
+            'name'                => '$promotedToo',
+            'content'             => 'ReadOnly private string|bool &$promotedToo',
+            'has_attributes'      => false,
+            'pass_by_reference'   => true,
+            'reference_token'     => 22, // Offset from the T_FUNCTION token.
+            'variable_length'     => false,
+            'variadic_token'      => false,
+            'type_hint'           => 'string|bool',
+            'type_hint_token'     => 18, // Offset from the T_FUNCTION token.
+            'type_hint_end_token' => 20, // Offset from the T_FUNCTION token.
+            'nullable_type'       => false,
+            'property_visibility' => 'private',
+            'visibility_token'    => 16, // Offset from the T_FUNCTION token.
+            'property_readonly'   => true,
+            'readonly_token'      => 14, // Offset from the T_FUNCTION token.
+            'comma_token'         => false,
+        ];
+
+        $this->getMethodParametersTestHelper('/* ' . __FUNCTION__ . ' */', $expected);
+    }
+
     /**
      * Verify behaviour when a non-constructor function uses PHP 8 property promotion syntax.
      *
@@ -1927,6 +1984,7 @@ public function testPHP8ConstructorPropertyPromotionGlobalFunction()
             'nullable_type'       => false,
             'property_visibility' => 'private',
             'visibility_token'    => 4, // Offset from the T_FUNCTION token.
+            'property_readonly'   => false,
             'comma_token'         => false,
         ];
 
@@ -1956,6 +2014,7 @@ public function testPHP8ConstructorPropertyPromotionAbstractMethod()
             'nullable_type'       => false,
             'property_visibility' => 'public',
             'visibility_token'    => 4, // Offset from the T_FUNCTION token.
+            'property_readonly'   => false,
             'comma_token'         => 9,
         ];
         $expected[1] = [
@@ -1973,6 +2032,7 @@ public function testPHP8ConstructorPropertyPromotionAbstractMethod()
             'nullable_type'       => false,
             'property_visibility' => 'private',
             'visibility_token'    => 11, // Offset from the T_FUNCTION token.
+            'property_readonly'   => false,
             'comma_token'         => false,
         ];
 
@@ -2036,6 +2096,7 @@ public function testParameterAttributesInFunctionDeclaration()
             'nullable_type'       => false,
             'property_visibility' => 'private',
             'visibility_token'    => ($php8Names === true) ? 10 : 13, // Offset from the T_FUNCTION token.
+            'property_readonly'   => false,
             'comma_token'         => ($php8Names === true) ? 15 : 18, // Offset from the T_FUNCTION token.
         ];
         $expected[1] = [
@@ -2390,6 +2451,9 @@ protected function getMethodParametersTestHelper($marker, $expected, $targetType
             if (isset($param['visibility_token'])) {
                 $expected[$key]['visibility_token'] += $target;
             }
+            if (isset($param['readonly_token'])) {
+                $expected[$key]['readonly_token'] += $target;
+            }
         }
 
         $this->assertSame($expected, $found);
diff --git a/Tests/Utils/FunctionDeclarations/GetParametersTest.php b/Tests/Utils/FunctionDeclarations/GetParametersTest.php
index b6c98683..92c6b353 100644
--- a/Tests/Utils/FunctionDeclarations/GetParametersTest.php
+++ b/Tests/Utils/FunctionDeclarations/GetParametersTest.php
@@ -143,6 +143,9 @@ protected function getMethodParametersTestHelper($marker, $expected, $targetType
             if (isset($param['visibility_token'])) {
                 $expected[$key]['visibility_token'] += $target;
             }
+            if (isset($param['readonly_token'])) {
+                $expected[$key]['readonly_token'] += $target;
+            }
         }
 
         $this->assertSame($expected, $found);