diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8af98eb37..74f44f037 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,10 @@ release-by-release.
 
 ## [Unreleased]
 
+### Added
+
+- **`AddSymfonyConstraintValidatorTypeDeclarationsRector`** — adds the Symfony 8 / Drupal 12 type declarations (`mixed $value` and `: void` on `validate()`, `: void` on `initialize()`) to `Symfony\Component\Validator\ConstraintValidatorInterface` implementers. Backward compatible on all supported Drupal versions, so no version gate. [#3600790]
+
 ## [1.0.0] — 2026-07-02
 
 ### Changed
diff --git a/config/drupal-11/drupal-11.0-deprecations.php b/config/drupal-11/drupal-11.0-deprecations.php
index a4d1e3ce7..97d3f2864 100644
--- a/config/drupal-11/drupal-11.0-deprecations.php
+++ b/config/drupal-11/drupal-11.0-deprecations.php
@@ -7,6 +7,7 @@
 use DrupalRector\Drupal11\Rector\Deprecation\MigrateSqlGetMigrationPluginManagerRector;
 use DrupalRector\Drupal11\Rector\Deprecation\RemoveStateCacheSettingRector;
 use DrupalRector\Drupal11\Rector\Deprecation\StripMigrationDependenciesExpandArgRector;
+use DrupalRector\Drupal12\Rector\Deprecation\AddSymfonyConstraintValidatorTypeDeclarationsRector;
 use DrupalRector\Rector\PHPUnit\PhpUnitTestAnnotationToAttributeRector;
 use DrupalRector\Rector\PHPUnit\ValueObject\PhpUnitTestAnnotationToAttributeConfiguration;
 use DrupalRector\Rector\ValueObject\DrupalIntroducedVersionConfiguration;
@@ -59,4 +60,12 @@
         new PhpUnitTestAnnotationToAttributeConfiguration('11.0.0', '12.0.0', 'depends', 'PHPUnit\Framework\Attributes\Depends'),
         new PhpUnitTestAnnotationToAttributeConfiguration('11.0.0', '12.0.0', 'testWith', 'PHPUnit\Framework\Attributes\TestWith'),
     ]);
+
+    // Forward-compat Symfony 8 / Drupal 12 signature change (NOT an 11.0
+    // deprecation). Registered here so it fires on drupal/core ^11.0 (every D11
+    // install) — the change is backward compatible, so applying it while still
+    // on D11 is safe and prepares the module for D12. Also registered in the
+    // drupal-12.0 set for installs already on ^12.0.
+    // https://git.drupalcode.org/project/redirect/-/merge_requests/200
+    $rectorConfig->rule(AddSymfonyConstraintValidatorTypeDeclarationsRector::class);
 };
diff --git a/config/drupal-12/drupal-12-all-deprecations.php b/config/drupal-12/drupal-12-all-deprecations.php
new file mode 100644
index 000000000..8f60c677e
--- /dev/null
+++ b/config/drupal-12/drupal-12-all-deprecations.php
@@ -0,0 +1,16 @@
+<?php
+
+declare(strict_types=1);
+
+use DrupalRector\Set\Drupal12SetList;
+use Rector\Config\RectorConfig;
+
+return static function (RectorConfig $rectorConfig): void {
+    $rectorConfig->sets([
+        Drupal12SetList::DRUPAL_120,
+    ]);
+
+    $rectorConfig->bootstrapFiles([
+        __DIR__.'/../drupal-phpunit-bootstrap-file.php',
+    ]);
+};
diff --git a/config/drupal-12/drupal-12.0-deprecations.php b/config/drupal-12/drupal-12.0-deprecations.php
new file mode 100644
index 000000000..f97c86108
--- /dev/null
+++ b/config/drupal-12/drupal-12.0-deprecations.php
@@ -0,0 +1,15 @@
+<?php
+
+declare(strict_types=1);
+
+use DrupalRector\Drupal12\Rector\Deprecation\AddSymfonyConstraintValidatorTypeDeclarationsRector;
+use Rector\Config\RectorConfig;
+
+return static function (RectorConfig $rectorConfig): void {
+    // Symfony 8 (Drupal 12) added native `: void` return types to
+    // ConstraintValidatorInterface::validate()/initialize() and `mixed $value`
+    // to validate() (the latter since Symfony 7). Add them to implementers.
+    // Backward compatible on all supported Drupal versions, so no version gate.
+    // https://git.drupalcode.org/project/redirect/-/merge_requests/200
+    $rectorConfig->rule(AddSymfonyConstraintValidatorTypeDeclarationsRector::class);
+};
diff --git a/src/Drupal12/Rector/Deprecation/AddSymfonyConstraintValidatorTypeDeclarationsRector.php b/src/Drupal12/Rector/Deprecation/AddSymfonyConstraintValidatorTypeDeclarationsRector.php
new file mode 100644
index 000000000..dac3164d5
--- /dev/null
+++ b/src/Drupal12/Rector/Deprecation/AddSymfonyConstraintValidatorTypeDeclarationsRector.php
@@ -0,0 +1,127 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DrupalRector\Drupal12\Rector\Deprecation;
+
+use PhpParser\Node;
+use PhpParser\Node\Identifier;
+use PhpParser\Node\Stmt\Class_;
+use PhpParser\Node\Stmt\ClassMethod;
+use PHPStan\Type\ObjectType;
+use Rector\Rector\AbstractRector;
+use Symplify\RuleDocGenerator\ValueObject\CodeSample\CodeSample;
+use Symplify\RuleDocGenerator\ValueObject\RuleDefinition;
+
+/**
+ * Adds the Symfony 8 / Drupal 12 type declarations to
+ * ConstraintValidatorInterface implementers.
+ *
+ * Symfony 7.0 (Drupal 11.0) added `mixed $value` to
+ * ConstraintValidatorInterface::validate(); Symfony 8.0 (Drupal 12.0) added
+ * native `: void` return types to validate() and initialize(). This rector adds
+ * those to any implementer (`extends ConstraintValidator`,
+ * `implements ConstraintValidatorInterface`, or the legacy
+ * `extends Constraint implements ConstraintValidatorInterface`).
+ *
+ * The change is backward compatible on every supported Drupal version: a child
+ * may declare types its interface does not (Drupal core itself ships
+ * `validate(mixed $value, Constraint $constraint): void` while running on a
+ * Symfony 7.4 interface without the native `: void`), so no BC wrapping or
+ * version gate is needed.
+ *
+ * @see https://git.drupalcode.org/project/redirect/-/merge_requests/200 (issue #3602388)
+ * @see https://github.com/symfony/symfony/blob/8.0/src/Symfony/Component/Validator/ConstraintValidatorInterface.php
+ */
+final class AddSymfonyConstraintValidatorTypeDeclarationsRector extends AbstractRector
+{
+    private const CONSTRAINT_VALIDATOR_INTERFACE = 'Symfony\Component\Validator\ConstraintValidatorInterface';
+
+    public function getRuleDefinition(): RuleDefinition
+    {
+        return new RuleDefinition(
+            'Add Symfony ConstraintValidatorInterface type declarations (mixed $value / : void) to validator classes.',
+            [
+                new CodeSample(
+                    <<<'CODE_BEFORE'
+use Symfony\Component\Validator\Constraint;
+use Symfony\Component\Validator\ConstraintValidator;
+
+class MyValidator extends ConstraintValidator {
+    public function validate($value, Constraint $constraint) {
+    }
+}
+CODE_BEFORE,
+                    <<<'CODE_AFTER'
+use Symfony\Component\Validator\Constraint;
+use Symfony\Component\Validator\ConstraintValidator;
+
+class MyValidator extends ConstraintValidator {
+    public function validate(mixed $value, Constraint $constraint): void {
+    }
+}
+CODE_AFTER
+                ),
+            ]
+        );
+    }
+
+    /** @return array<class-string<Node>> */
+    public function getNodeTypes(): array
+    {
+        return [Class_::class];
+    }
+
+    /**
+     * @param Class_ $node
+     */
+    public function refactor(Node $node): ?Node
+    {
+        if (!$this->isObjectType($node, new ObjectType(self::CONSTRAINT_VALIDATOR_INTERFACE))) {
+            return null;
+        }
+
+        $changed = false;
+
+        $validate = $node->getMethod('validate');
+        if ($validate instanceof ClassMethod) {
+            if ($this->addMixedFirstParam($validate)) {
+                $changed = true;
+            }
+            if ($this->addVoidReturnType($validate)) {
+                $changed = true;
+            }
+        }
+
+        $initialize = $node->getMethod('initialize');
+        if ($initialize instanceof ClassMethod) {
+            if ($this->addVoidReturnType($initialize)) {
+                $changed = true;
+            }
+        }
+
+        return $changed ? $node : null;
+    }
+
+    private function addVoidReturnType(ClassMethod $method): bool
+    {
+        if ($method->returnType !== null) {
+            return false;
+        }
+
+        $method->returnType = new Identifier('void');
+
+        return true;
+    }
+
+    private function addMixedFirstParam(ClassMethod $method): bool
+    {
+        if (!isset($method->params[0]) || $method->params[0]->type !== null) {
+            return false;
+        }
+
+        $method->params[0]->type = new Identifier('mixed');
+
+        return true;
+    }
+}
diff --git a/src/Set/Drupal12SetList.php b/src/Set/Drupal12SetList.php
new file mode 100644
index 000000000..f47d7768e
--- /dev/null
+++ b/src/Set/Drupal12SetList.php
@@ -0,0 +1,11 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DrupalRector\Set;
+
+final class Drupal12SetList
+{
+    public const DRUPAL_12 = __DIR__.'/../../config/drupal-12/drupal-12-all-deprecations.php';
+    public const DRUPAL_120 = __DIR__.'/../../config/drupal-12/drupal-12.0-deprecations.php';
+}
diff --git a/src/Set/DrupalSetProvider.php b/src/Set/DrupalSetProvider.php
index ea35c028c..cfcfc94e6 100644
--- a/src/Set/DrupalSetProvider.php
+++ b/src/Set/DrupalSetProvider.php
@@ -51,6 +51,7 @@ final class DrupalSetProvider implements SetProviderInterface
         '11.2' => Drupal11SetList::DRUPAL_112,
         '11.3' => Drupal11SetList::DRUPAL_113,
         '11.4' => Drupal11SetList::DRUPAL_114,
+        '12.0' => Drupal12SetList::DRUPAL_120,
     ];
 
     /**
@@ -74,7 +75,7 @@ final class DrupalSetProvider implements SetProviderInterface
      *
      * @var string[]
      */
-    private const MAJOR_FLOORS = ['10.0', '11.0'];
+    private const MAJOR_FLOORS = ['10.0', '11.0', '12.0'];
 
     /**
      * @return SetInterface[]
diff --git a/stubs/Symfony/Component/Validator/Constraint.php b/stubs/Symfony/Component/Validator/Constraint.php
new file mode 100644
index 000000000..de7ed93ac
--- /dev/null
+++ b/stubs/Symfony/Component/Validator/Constraint.php
@@ -0,0 +1,13 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Symfony\Component\Validator;
+
+if (class_exists(\Symfony\Component\Validator\Constraint::class)) {
+    return;
+}
+
+abstract class Constraint
+{
+}
diff --git a/stubs/Symfony/Component/Validator/ConstraintValidator.php b/stubs/Symfony/Component/Validator/ConstraintValidator.php
new file mode 100644
index 000000000..fca2fdfa0
--- /dev/null
+++ b/stubs/Symfony/Component/Validator/ConstraintValidator.php
@@ -0,0 +1,22 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Symfony\Component\Validator;
+
+use Symfony\Component\Validator\Context\ExecutionContextInterface;
+
+if (class_exists(\Symfony\Component\Validator\ConstraintValidator::class)) {
+    return;
+}
+
+abstract class ConstraintValidator implements ConstraintValidatorInterface
+{
+    public function initialize(ExecutionContextInterface $context): void
+    {
+    }
+
+    public function validate(mixed $value, Constraint $constraint)
+    {
+    }
+}
diff --git a/stubs/Symfony/Component/Validator/ConstraintValidatorInterface.php b/stubs/Symfony/Component/Validator/ConstraintValidatorInterface.php
new file mode 100644
index 000000000..53bd9c5d9
--- /dev/null
+++ b/stubs/Symfony/Component/Validator/ConstraintValidatorInterface.php
@@ -0,0 +1,18 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Symfony\Component\Validator;
+
+use Symfony\Component\Validator\Context\ExecutionContextInterface;
+
+if (interface_exists(\Symfony\Component\Validator\ConstraintValidatorInterface::class)) {
+    return;
+}
+
+interface ConstraintValidatorInterface
+{
+    public function initialize(ExecutionContextInterface $context);
+
+    public function validate(mixed $value, Constraint $constraint);
+}
diff --git a/stubs/Symfony/Component/Validator/Context/ExecutionContextInterface.php b/stubs/Symfony/Component/Validator/Context/ExecutionContextInterface.php
new file mode 100644
index 000000000..5ff2ddce4
--- /dev/null
+++ b/stubs/Symfony/Component/Validator/Context/ExecutionContextInterface.php
@@ -0,0 +1,13 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Symfony\Component\Validator\Context;
+
+if (interface_exists(\Symfony\Component\Validator\Context\ExecutionContextInterface::class)) {
+    return;
+}
+
+interface ExecutionContextInterface
+{
+}
diff --git a/tests/src/Drupal12/Rector/Deprecation/AddSymfonyConstraintValidatorTypeDeclarationsRector/AddSymfonyConstraintValidatorTypeDeclarationsRectorTest.php b/tests/src/Drupal12/Rector/Deprecation/AddSymfonyConstraintValidatorTypeDeclarationsRector/AddSymfonyConstraintValidatorTypeDeclarationsRectorTest.php
new file mode 100644
index 000000000..f356dc1bf
--- /dev/null
+++ b/tests/src/Drupal12/Rector/Deprecation/AddSymfonyConstraintValidatorTypeDeclarationsRector/AddSymfonyConstraintValidatorTypeDeclarationsRectorTest.php
@@ -0,0 +1,26 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DrupalRector\Tests\Drupal12\Rector\Deprecation\AddSymfonyConstraintValidatorTypeDeclarationsRector;
+
+use DrupalRector\Tests\AbstractDrupalRectorTestCase;
+
+class AddSymfonyConstraintValidatorTypeDeclarationsRectorTest extends AbstractDrupalRectorTestCase
+{
+    #[\PHPUnit\Framework\Attributes\DataProvider('provideData')]
+    public function test(string $filePath): void
+    {
+        $this->doTestFile($filePath);
+    }
+
+    public static function provideData(): \Iterator
+    {
+        return self::yieldFilesFromDirectory(__DIR__.'/fixture');
+    }
+
+    public function provideConfigFilePath(): string
+    {
+        return __DIR__.'/config/configured_rule.php';
+    }
+}
diff --git a/tests/src/Drupal12/Rector/Deprecation/AddSymfonyConstraintValidatorTypeDeclarationsRector/config/configured_rule.php b/tests/src/Drupal12/Rector/Deprecation/AddSymfonyConstraintValidatorTypeDeclarationsRector/config/configured_rule.php
new file mode 100644
index 000000000..97029ca02
--- /dev/null
+++ b/tests/src/Drupal12/Rector/Deprecation/AddSymfonyConstraintValidatorTypeDeclarationsRector/config/configured_rule.php
@@ -0,0 +1,11 @@
+<?php
+
+declare(strict_types=1);
+
+use DrupalRector\Drupal12\Rector\Deprecation\AddSymfonyConstraintValidatorTypeDeclarationsRector;
+use DrupalRector\Tests\Rector\Deprecation\DeprecationBase;
+use Rector\Config\RectorConfig;
+
+return static function (RectorConfig $rectorConfig): void {
+    DeprecationBase::addClass(AddSymfonyConstraintValidatorTypeDeclarationsRector::class, $rectorConfig, false);
+};
diff --git a/tests/src/Drupal12/Rector/Deprecation/AddSymfonyConstraintValidatorTypeDeclarationsRector/fixture/already_typed.php.inc b/tests/src/Drupal12/Rector/Deprecation/AddSymfonyConstraintValidatorTypeDeclarationsRector/fixture/already_typed.php.inc
new file mode 100644
index 000000000..82c86e6bc
--- /dev/null
+++ b/tests/src/Drupal12/Rector/Deprecation/AddSymfonyConstraintValidatorTypeDeclarationsRector/fixture/already_typed.php.inc
@@ -0,0 +1,14 @@
+<?php
+
+namespace Drupal\mymodule\Plugin\Validation\Constraint;
+
+use Symfony\Component\Validator\Constraint;
+use Symfony\Component\Validator\ConstraintValidator;
+
+class AlreadyTypedValidator extends ConstraintValidator {
+
+    public function validate(mixed $value, Constraint $constraint): void {
+    }
+
+}
+?>
diff --git a/tests/src/Drupal12/Rector/Deprecation/AddSymfonyConstraintValidatorTypeDeclarationsRector/fixture/constraint_only.php.inc b/tests/src/Drupal12/Rector/Deprecation/AddSymfonyConstraintValidatorTypeDeclarationsRector/fixture/constraint_only.php.inc
new file mode 100644
index 000000000..1a2e1d8b0
--- /dev/null
+++ b/tests/src/Drupal12/Rector/Deprecation/AddSymfonyConstraintValidatorTypeDeclarationsRector/fixture/constraint_only.php.inc
@@ -0,0 +1,13 @@
+<?php
+
+namespace Drupal\mymodule\Plugin\Validation\Constraint;
+
+use Symfony\Component\Validator\Constraint;
+
+class SomeConstraint extends Constraint {
+
+    public function validate($value, Constraint $constraint) {
+    }
+
+}
+?>
diff --git a/tests/src/Drupal12/Rector/Deprecation/AddSymfonyConstraintValidatorTypeDeclarationsRector/fixture/constraint_validator.php.inc b/tests/src/Drupal12/Rector/Deprecation/AddSymfonyConstraintValidatorTypeDeclarationsRector/fixture/constraint_validator.php.inc
new file mode 100644
index 000000000..04f79a3c5
--- /dev/null
+++ b/tests/src/Drupal12/Rector/Deprecation/AddSymfonyConstraintValidatorTypeDeclarationsRector/fixture/constraint_validator.php.inc
@@ -0,0 +1,29 @@
+<?php
+
+namespace Drupal\mymodule\Plugin\Validation\Constraint;
+
+use Symfony\Component\Validator\Constraint;
+use Symfony\Component\Validator\ConstraintValidator;
+
+class UniqueHashValidator extends ConstraintValidator {
+
+    public function validate($redirect, Constraint $constraint) {
+    }
+
+}
+?>
+-----
+<?php
+
+namespace Drupal\mymodule\Plugin\Validation\Constraint;
+
+use Symfony\Component\Validator\Constraint;
+use Symfony\Component\Validator\ConstraintValidator;
+
+class UniqueHashValidator extends ConstraintValidator {
+
+    public function validate(mixed $redirect, Constraint $constraint): void {
+    }
+
+}
+?>
diff --git a/tests/src/Drupal12/Rector/Deprecation/AddSymfonyConstraintValidatorTypeDeclarationsRector/fixture/not_a_validator.php.inc b/tests/src/Drupal12/Rector/Deprecation/AddSymfonyConstraintValidatorTypeDeclarationsRector/fixture/not_a_validator.php.inc
new file mode 100644
index 000000000..fc8076f7a
--- /dev/null
+++ b/tests/src/Drupal12/Rector/Deprecation/AddSymfonyConstraintValidatorTypeDeclarationsRector/fixture/not_a_validator.php.inc
@@ -0,0 +1,11 @@
+<?php
+
+namespace Drupal\mymodule;
+
+class SomeService {
+
+    public function validate($value, $constraint) {
+    }
+
+}
+?>
diff --git a/tests/src/Drupal12/Rector/Deprecation/AddSymfonyConstraintValidatorTypeDeclarationsRector/fixture/self_validating_constraint.php.inc b/tests/src/Drupal12/Rector/Deprecation/AddSymfonyConstraintValidatorTypeDeclarationsRector/fixture/self_validating_constraint.php.inc
new file mode 100644
index 000000000..a0f6587a1
--- /dev/null
+++ b/tests/src/Drupal12/Rector/Deprecation/AddSymfonyConstraintValidatorTypeDeclarationsRector/fixture/self_validating_constraint.php.inc
@@ -0,0 +1,37 @@
+<?php
+
+namespace Drupal\mymodule\Plugin\Validation\Constraint;
+
+use Symfony\Component\Validator\Constraint;
+use Symfony\Component\Validator\ConstraintValidatorInterface;
+use Symfony\Component\Validator\Context\ExecutionContextInterface;
+
+class SourceLinkTypeConstraint extends Constraint implements ConstraintValidatorInterface {
+
+    public function initialize(ExecutionContextInterface $context) {
+    }
+
+    public function validate($value, Constraint $constraint) {
+    }
+
+}
+?>
+-----
+<?php
+
+namespace Drupal\mymodule\Plugin\Validation\Constraint;
+
+use Symfony\Component\Validator\Constraint;
+use Symfony\Component\Validator\ConstraintValidatorInterface;
+use Symfony\Component\Validator\Context\ExecutionContextInterface;
+
+class SourceLinkTypeConstraint extends Constraint implements ConstraintValidatorInterface {
+
+    public function initialize(ExecutionContextInterface $context): void {
+    }
+
+    public function validate(mixed $value, Constraint $constraint): void {
+    }
+
+}
+?>
diff --git a/tests/src/Set/DrupalSetProviderTest.php b/tests/src/Set/DrupalSetProviderTest.php
index b95145bd1..2bec774f6 100644
--- a/tests/src/Set/DrupalSetProviderTest.php
+++ b/tests/src/Set/DrupalSetProviderTest.php
@@ -90,8 +90,11 @@ public static function installedVersionProvider(): array
                 'drupal-11.4-breaking.php',
                 'drupal-bootstrap.php',
             ]],
-            // A future major out of range matches nothing.
-            '12.0.0 matches nothing' => ['12.0.0', []],
+            // The next major's floor loads only its own set + bootstrap.
+            '12.0.0 loads only 12.0' => ['12.0.0', [
+                'drupal-12.0-deprecations.php',
+                'drupal-bootstrap.php',
+            ]],
         ];
     }