diff --git a/README.md b/README.md
index c5485051..370956e5 100644
--- a/README.md
+++ b/README.md
@@ -67,6 +67,7 @@ Polyfills are provided for:
 - the `str_increment` and `str_decrement` functions introduced in PHP 8.3;
 - the `Date*Exception/Error` classes introduced in PHP 8.3;
 - the `SQLite3Exception` class introduced in PHP 8.3;
+- the `mb_ucfirst` and `mb_lcfirst` functions introduced in PHP 8.4;
 
 It is strongly recommended to upgrade your PHP version and/or install the missing
 extensions whenever possible. This polyfill should be used only when there is no
diff --git a/src/Mbstring/Mbstring.php b/src/Mbstring/Mbstring.php
index 04b35cb8..bccd8318 100644
--- a/src/Mbstring/Mbstring.php
+++ b/src/Mbstring/Mbstring.php
@@ -48,6 +48,8 @@
  * - mb_strstr               - Finds first occurrence of a string within another
  * - mb_strwidth             - Return width of string
  * - mb_substr_count         - Count the number of substring occurrences
+ * - mb_ucfirst              - Make a string's first character uppercase
+ * - mb_lcfirst              - Make a string's first character lowercase
  *
  * Not implemented:
  * - mb_convert_kana         - Convert "kana" one from another ("zen-kaku", "han-kaku" and more)
@@ -871,6 +873,16 @@ public static function mb_str_pad(string $string, int $length, string $pad_strin
         }
     }
 
+    public static function mb_ucfirst(string $string, ?string $encoding = null): string
+    {
+        return \mb_convert_case(\mb_substr($string, 0, 1, $encoding), \MB_CASE_TITLE, $encoding) . \mb_substr($string, 1, null, $encoding);
+    }
+
+    public static function mb_lcfirst(string $string, ?string $encoding = null): string
+    {
+        return \mb_strtolower(\mb_substr($string, 0, 1, $encoding), $encoding) . \mb_substr($string, 1, null, $encoding);
+    }
+
     private static function getSubpart($pos, $part, $haystack, $encoding)
     {
         if (false === $pos) {
diff --git a/src/Mbstring/bootstrap.php b/src/Mbstring/bootstrap.php
index ecf1a035..ef8a97f8 100644
--- a/src/Mbstring/bootstrap.php
+++ b/src/Mbstring/bootstrap.php
@@ -136,6 +136,13 @@ function mb_str_split($string, $length = 1, $encoding = null) { return p\Mbstrin
     function mb_str_pad(string $string, int $length, string $pad_string = ' ', int $pad_type = STR_PAD_RIGHT, ?string $encoding = null): string { return p\Mbstring::mb_str_pad($string, $length, $pad_string, $pad_type, $encoding); }
 }
 
+if (!function_exists('mb_ucfirst')) {
+    function mb_ucfirst(string $string, ?string $encoding = null): string { return p\Mbstring::mb_ucfirst($string, $encoding); }
+}
+if (!function_exists('mb_lcfirst'))  {
+    function mb_lcfirst(string $string, ?string $encoding = null): string { return p\Mbstring::mb_lcfirst($string, $encoding); }
+}
+
 if (extension_loaded('mbstring')) {
     return;
 }
diff --git a/src/Mbstring/bootstrap80.php b/src/Mbstring/bootstrap80.php
index 2f9fb5b4..c553cb34 100644
--- a/src/Mbstring/bootstrap80.php
+++ b/src/Mbstring/bootstrap80.php
@@ -132,6 +132,14 @@ function mb_str_split(?string $string, ?int $length = 1, ?string $encoding = nul
     function mb_str_pad(string $string, int $length, string $pad_string = ' ', int $pad_type = STR_PAD_RIGHT, ?string $encoding = null): string { return p\Mbstring::mb_str_pad($string, $length, $pad_string, $pad_type, $encoding); }
 }
 
+if (!function_exists('mb_ucfirst')) {
+    function mb_ucfirst(string $string, ?string $encoding = null): string { return p\Mbstring::mb_ucfirst($string, $encoding); }
+}
+
+if (!function_exists('mb_lcfirst'))  {
+    function mb_lcfirst(string $string, ?string $encoding = null): string { return p\Mbstring::mb_lcfirst($string, $encoding); }
+}
+
 if (extension_loaded('mbstring')) {
     return;
 }
diff --git a/src/Php84/Php84.php b/src/Php84/Php84.php
index dc8eea84..f80b71ff 100644
--- a/src/Php84/Php84.php
+++ b/src/Php84/Php84.php
@@ -18,4 +18,13 @@
  */
 final class Php84
 {
+    public static function mb_ucfirst(string $string, ?string $encoding = null): string
+    {
+        return \mb_convert_case(\mb_substr($string, 0, 1, $encoding), \MB_CASE_TITLE, $encoding) . \mb_substr($string, 1, null, $encoding);
+    }
+
+    public static function mb_lcfirst(string $string, ?string $encoding = null): string
+    {
+        return \mb_strtolower(\mb_substr($string, 0, 1, $encoding), $encoding) . \mb_substr($string, 1, null, $encoding);
+    }
 }
diff --git a/src/Php84/README.md b/src/Php84/README.md
index af10869c..e94050d9 100644
--- a/src/Php84/README.md
+++ b/src/Php84/README.md
@@ -3,6 +3,9 @@ Symfony Polyfill / Php84
 
 This component provides features added to PHP 8.4 core:
 
+- [`mb_lcfirst`](https://wiki.php.net/rfc/mb_ucfirst)
+- [`mb_ucfirst`](https://wiki.php.net/rfc/mb_ucfirst)
+
 More information can be found in the
 [main Polyfill README](https://github.com/symfony/polyfill/blob/main/README.md).
 
diff --git a/src/Php84/bootstrap.php b/src/Php84/bootstrap.php
index a0371b29..bd16633e 100644
--- a/src/Php84/bootstrap.php
+++ b/src/Php84/bootstrap.php
@@ -14,3 +14,11 @@
 if (\PHP_VERSION_ID >= 80400) {
     return;
 }
+
+if (!function_exists('mb_ucfirst') && function_exists('mb_substr')) {
+    function mb_ucfirst(string $string, ?string $encoding = null): string { return p\Php84::mb_ucfirst($string, $encoding); }
+}
+
+if (!function_exists('mb_lcfirst') && function_exists('mb_substr'))  {
+    function mb_lcfirst(string $string, ?string $encoding = null): string { return p\Php84::mb_lcfirst($string, $encoding); }
+}