diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
index 0000000..7ed1bed
--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,7 @@
+# Changelog
+
+## [Unreleased]
+
+### Added
+
+- `Innmind\Validation\Shape::optional()`
diff --git a/proofs/shape.php b/proofs/shape.php
index c640451..0998173 100644
--- a/proofs/shape.php
+++ b/proofs/shape.php
@@ -103,4 +103,33 @@ static function($assert) {
             );
         },
     );
+
+    yield test(
+        'Shape with optional key',
+        static function($assert) {
+            $assert->true(
+                Shape::of('foo', Is::int())
+                    ->with('bar', Is::bool())
+                    ->optional('bar')
+                    ->asPredicate()([
+                        'foo' => 42,
+                    ]),
+            );
+            $assert->same(
+                [
+                    'foo' => 42,
+                ],
+                Shape::of('foo', Is::int())
+                    ->with('bar', Is::bool())
+                    ->optional('bar')([
+                        'foo' => 42,
+                        'baz' => 'invalid',
+                    ])
+                    ->match(
+                        static fn($value) => $value,
+                        static fn() => null,
+                    ),
+            );
+        },
+    );
 };
diff --git a/src/Shape.php b/src/Shape.php
index 0877d09..96f4eba 100644
--- a/src/Shape.php
+++ b/src/Shape.php
@@ -16,32 +16,51 @@ final class Shape implements Constraint
 {
     /** @var non-empty-array<non-empty-string, Constraint<mixed, mixed>> */
     private array $constraints;
+    /** @var list<non-empty-string> */
+    private array $optional;
 
     /**
      * @param non-empty-array<non-empty-string, Constraint<mixed, mixed>> $constraints
+     * @param list<non-empty-string> $optional
      */
-    private function __construct(array $constraints)
+    private function __construct(array $constraints, array $optional)
     {
         $this->constraints = $constraints;
+        $this->optional = $optional;
     }
 
     public function __invoke(mixed $value): Validation
     {
+        $optional = new \stdClass;
         /** @var Validation<Failure, non-empty-array<non-empty-string, mixed>> */
         $validation = Validation::success([]);
 
         foreach ($this->constraints as $key => $constraint) {
+            $keyValidation = Has::key($key);
+
+            if (\in_array($key, $this->optional, true)) {
+                /** @psalm-suppress MixedArgumentTypeCoercion */
+                $keyValidation = $keyValidation->or(Of::callable(
+                    static fn() => Validation::success($optional),
+                ));
+            }
+
             $ofType = Of::callable(
-                static fn($value) => $constraint($value)->mapFailures(
-                    static fn($failure) => $failure->under($key),
-                ),
+                static fn($value) => match ($value) {
+                    $optional => Validation::success($optional),
+                    default => $constraint($value)->mapFailures(
+                        static fn($failure) => $failure->under($key),
+                    ),
+                },
             );
 
             $validation = $validation->and(
-                Has::key($key)->and($ofType)($value),
-                static function($array, $value) use ($key) {
-                    /** @psalm-suppress MixedAssignment */
-                    $array[$key] = $value;
+                $keyValidation->and($ofType)($value),
+                static function($array, $value) use ($key, $optional) {
+                    if ($value !== $optional) {
+                        /** @psalm-suppress MixedAssignment */
+                        $array[$key] = $value;
+                    }
 
                     return $array;
                 },
@@ -58,7 +77,7 @@ static function($array, $value) use ($key) {
      */
     public static function of(string $key, Constraint $constraint): self
     {
-        return new self([$key => $constraint]);
+        return new self([$key => $constraint], []);
     }
 
     /**
@@ -69,7 +88,18 @@ public function with(string $key, Constraint $constraint): self
         $constraints = $this->constraints;
         $constraints[$key] = $constraint;
 
-        return new self($constraints);
+        return new self($constraints, $this->optional);
+    }
+
+    /**
+     * @param non-empty-string $key
+     */
+    public function optional(string $key): self
+    {
+        $optional = $this->optional;
+        $optional[] = $key;
+
+        return new self($this->constraints, $optional);
     }
 
     public function and(Constraint $constraint): Constraint