diff --git a/extension.neon b/extension.neon
index 6d50e3c5..fe294170 100644
--- a/extension.neon
+++ b/extension.neon
@@ -37,6 +37,7 @@ parameters:
 			hookRules: false
 			loggerFromFactoryPropertyAssignmentRule: false
 			entityStorageDirectInjectionRule: false
+			constraintValidatorMustNarrowConstraintType: false
 		entityMapping:
 			aggregator_feed:
 				class: Drupal\aggregator\Entity\Feed
@@ -275,6 +276,7 @@ parametersSchema:
 			hookRules: boolean()
 			loggerFromFactoryPropertyAssignmentRule: boolean()
 			entityStorageDirectInjectionRule: boolean()
+			constraintValidatorMustNarrowConstraintType: boolean()
 		])
 		entityMapping: arrayOf(anyOf(
 			structure([
diff --git a/rules.neon b/rules.neon
index b66d6497..0c927286 100644
--- a/rules.neon
+++ b/rules.neon
@@ -34,6 +34,8 @@ conditionalTags:
 		phpstan.rules.rule: %drupal.rules.entityStorageDirectInjectionRule%
 	mglaman\PHPStanDrupal\Rules\Drupal\EntityStoragePropertyAssignmentRule:
 		phpstan.rules.rule: %drupal.rules.entityStorageDirectInjectionRule%
+	mglaman\PHPStanDrupal\Rules\Drupal\ConstraintValidatorValidateNarrowsConstraintTypeRule:
+		phpstan.rules.rule: %drupal.rules.constraintValidatorMustNarrowConstraintType%
 
 services:
 	-
@@ -58,3 +60,5 @@ services:
 		class: mglaman\PHPStanDrupal\Rules\Drupal\EntityStorageDirectInjectionRule
 	-
 		class: mglaman\PHPStanDrupal\Rules\Drupal\EntityStoragePropertyAssignmentRule
+	-
+		class: mglaman\PHPStanDrupal\Rules\Drupal\ConstraintValidatorValidateNarrowsConstraintTypeRule
diff --git a/src/Rules/Drupal/ConstraintValidatorValidateNarrowsConstraintTypeRule.php b/src/Rules/Drupal/ConstraintValidatorValidateNarrowsConstraintTypeRule.php
new file mode 100644
index 00000000..e7c54ae5
--- /dev/null
+++ b/src/Rules/Drupal/ConstraintValidatorValidateNarrowsConstraintTypeRule.php
@@ -0,0 +1,161 @@
+<?php
+
+declare(strict_types=1);
+
+namespace mglaman\PHPStanDrupal\Rules\Drupal;
+
+use PhpParser\Node;
+use PHPStan\Analyser\Scope;
+use PHPStan\Reflection\ReflectionProvider;
+use PHPStan\Rules\Rule;
+use PHPStan\Rules\RuleErrorBuilder;
+use Symfony\Component\Validator\Constraint;
+use Symfony\Component\Validator\ConstraintValidator;
+use function array_key_exists;
+use function count;
+use function str_ends_with;
+use function strrpos;
+use function substr;
+
+/**
+ * Requires ConstraintValidator::validate() to narrow the $constraint parameter type.
+ *
+ * PHPStan cannot infer that `$constraint` is a specific subclass of Constraint
+ * inside a validator's validate() method. Drupal core's convention is to use
+ * `assert($constraint instanceof SpecificConstraint)` to narrow the type, which
+ * PHPStan handles natively.
+ *
+ * This rule fires when a class following the FooValidator/Foo naming convention
+ * does not assert the constraint type in its validate() method.
+ *
+ * @implements Rule<Node\Stmt\ClassMethod>
+ */
+final class ConstraintValidatorValidateNarrowsConstraintTypeRule implements Rule
+{
+
+    private ReflectionProvider $reflectionProvider;
+
+    public function __construct(ReflectionProvider $reflectionProvider)
+    {
+        $this->reflectionProvider = $reflectionProvider;
+    }
+
+    public function getNodeType(): string
+    {
+        return Node\Stmt\ClassMethod::class;
+    }
+
+    public function processNode(Node $node, Scope $scope): array
+    {
+        if ($node->name->toString() !== 'validate') {
+            return [];
+        }
+        if (!$scope->isInClass()) {
+            return [];
+        }
+
+        // Guard: symfony/validator may not be installed in every project.
+        if (!$this->reflectionProvider->hasClass(ConstraintValidator::class)
+            || !$this->reflectionProvider->hasClass(Constraint::class)) {
+            return [];
+        }
+
+        $classReflection = $scope->getClassReflection();
+        if (!$classReflection->isSubclassOfClass($this->reflectionProvider->getClass(ConstraintValidator::class))) {
+            return [];
+        }
+
+        // Derive the matching constraint class name by stripping 'Validator' suffix.
+        $fqcn = $classReflection->getName();
+        if (!str_ends_with($fqcn, 'Validator')) {
+            return [];
+        }
+        $constraintClass = substr($fqcn, 0, -9);
+
+        if (!$this->reflectionProvider->hasClass($constraintClass)) {
+            return [];
+        }
+        $constraintReflection = $this->reflectionProvider->getClass($constraintClass);
+        if (!$constraintReflection->isSubclassOfClass($this->reflectionProvider->getClass(Constraint::class))) {
+            return [];
+        }
+
+        // Find the $constraint parameter name (second param by Symfony convention).
+        $constraintParamName = 'constraint';
+        if (array_key_exists(1, $node->params)) {
+            $param = $node->params[1];
+            if ($param->var instanceof Node\Expr\Variable && is_string($param->var->name)) {
+                $constraintParamName = $param->var->name;
+            }
+        }
+
+        // Check if the method body asserts the concrete constraint type.
+        if ($this->hasConstraintAssertion($node->stmts ?? [], $constraintParamName, $constraintClass, $scope)) {
+            return [];
+        }
+
+        $validatorFqcn = $classReflection->getName();
+        $validatorPos = strrpos($validatorFqcn, '\\');
+        $validatorShortName = $validatorPos !== false ? substr($validatorFqcn, $validatorPos + 1) : $validatorFqcn;
+        $constraintFqcn = $constraintReflection->getName();
+        $constraintPos = strrpos($constraintFqcn, '\\');
+        $constraintShortName = $constraintPos !== false ? substr($constraintFqcn, $constraintPos + 1) : $constraintFqcn;
+
+        return [
+            RuleErrorBuilder::message(sprintf(
+                'Method %s::validate() does not narrow the $%s parameter type. Add assert($%s instanceof %s) to allow PHPStan to infer constraint-specific properties.',
+                $validatorShortName,
+                $constraintParamName,
+                $constraintParamName,
+                $constraintShortName
+            ))
+            ->identifier('drupal.constraintValidator.missingNarrow')
+            ->tip('See https://www.drupal.org/project/drupal/issues/3246287')
+            ->build(),
+        ];
+    }
+
+    /**
+     * Returns true if the statement list contains assert($var instanceof ExpectedConstraintClass).
+     *
+     * @param Node\Stmt[] $stmts
+     */
+    private function hasConstraintAssertion(array $stmts, string $paramName, string $expectedConstraintFqcn, Scope $scope): bool
+    {
+        foreach ($stmts as $stmt) {
+            if (!$stmt instanceof Node\Stmt\Expression) {
+                continue;
+            }
+            $expr = $stmt->expr;
+            if (!$expr instanceof Node\Expr\FuncCall) {
+                continue;
+            }
+            if (!$expr->name instanceof Node\Name || $expr->name->toString() !== 'assert') {
+                continue;
+            }
+            $args = $expr->getArgs();
+            if (count($args) === 0) {
+                continue;
+            }
+            $assertArg = $args[0]->value;
+            if (!$assertArg instanceof Node\Expr\Instanceof_) {
+                continue;
+            }
+            if (!$assertArg->expr instanceof Node\Expr\Variable) {
+                continue;
+            }
+            if ($assertArg->expr->name !== $paramName) {
+                continue;
+            }
+            if (!$assertArg->class instanceof Node\Name) {
+                continue;
+            }
+            $assertedClass = $scope->resolveName($assertArg->class);
+            if ($assertedClass === $expectedConstraintFqcn) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+}
diff --git a/tests/src/Rules/ConstraintValidatorValidateNarrowsConstraintTypeRuleTest.php b/tests/src/Rules/ConstraintValidatorValidateNarrowsConstraintTypeRuleTest.php
new file mode 100644
index 00000000..dd6a4457
--- /dev/null
+++ b/tests/src/Rules/ConstraintValidatorValidateNarrowsConstraintTypeRuleTest.php
@@ -0,0 +1,33 @@
+<?php
+
+declare(strict_types=1);
+
+namespace mglaman\PHPStanDrupal\Tests\Rules;
+
+use mglaman\PHPStanDrupal\Rules\Drupal\ConstraintValidatorValidateNarrowsConstraintTypeRule;
+use mglaman\PHPStanDrupal\Tests\DrupalRuleTestCase;
+use PHPStan\Rules\Rule;
+
+final class ConstraintValidatorValidateNarrowsConstraintTypeRuleTest extends DrupalRuleTestCase
+{
+
+    protected function getRule(): Rule
+    {
+        return self::getContainer()->getByType(ConstraintValidatorValidateNarrowsConstraintTypeRule::class);
+    }
+
+    public function testRule(): void
+    {
+        $this->analyse(
+            [__DIR__ . '/data/constraint-validator.php'],
+            [
+                [
+                    'Method UniqueItemValidator::validate() does not narrow the $constraint parameter type. Add assert($constraint instanceof UniqueItem) to allow PHPStan to infer constraint-specific properties.',
+                    22,
+                    'See https://www.drupal.org/project/drupal/issues/3246287',
+                ],
+            ]
+        );
+    }
+
+}
diff --git a/tests/src/Rules/data/constraint-validator.php b/tests/src/Rules/data/constraint-validator.php
new file mode 100644
index 00000000..9381b845
--- /dev/null
+++ b/tests/src/Rules/data/constraint-validator.php
@@ -0,0 +1,70 @@
+<?php
+
+declare(strict_types=1);
+
+namespace mglaman\PHPStanDrupal\Tests\Rules\data\ConstraintValidator;
+
+use Symfony\Component\Validator\Constraint;
+use Symfony\Component\Validator\ConstraintValidator;
+
+// Constraint class following the naming convention: UniqueItem / UniqueItemValidator.
+class UniqueItem extends Constraint
+{
+
+    public string $alreadyExists = 'The item %value already exists.';
+
+}
+
+// Error: validate() does not assert the concrete constraint type.
+class UniqueItemValidator extends ConstraintValidator
+{
+
+    public function validate(mixed $value, Constraint $constraint): void
+    {
+        $this->context->addViolation($constraint->alreadyExists);
+    }
+
+}
+
+// Constraint class for the assert-detection test case.
+class UniqueItemWithAssert extends Constraint
+{
+
+    public string $alreadyExists = 'The item %value already exists.';
+
+}
+
+// No error: validate() uses assert() to narrow to the expected constraint type.
+class UniqueItemWithAssertValidator extends ConstraintValidator
+{
+
+    public function validate(mixed $value, Constraint $constraint): void
+    {
+        assert($constraint instanceof UniqueItemWithAssert);
+        $this->context->addViolation($constraint->alreadyExists);
+    }
+
+}
+
+// No error: 'StandaloneValidator' strips to 'Standalone', but there is no
+// corresponding constraint class with that name.
+class StandaloneValidator extends ConstraintValidator
+{
+
+    public function validate(mixed $value, Constraint $constraint): void
+    {
+        $this->context->addViolation('some message');
+    }
+
+}
+
+// No error: 'OrphanedValidator' strips to 'Orphaned', which is not a known constraint.
+class OrphanedValidator extends ConstraintValidator
+{
+
+    public function validate(mixed $value, Constraint $constraint): void
+    {
+        $this->context->addViolation('some message');
+    }
+
+}