diff --git a/src/Entity/Bucket.php b/src/Entity/Bucket.php
index 6d6811d..4ce047f 100644
--- a/src/Entity/Bucket.php
+++ b/src/Entity/Bucket.php
@@ -6,6 +6,7 @@ use App\Enum\BucketAllocationType;
 use App\Enum\BucketType;
 use App\Repository\BucketRepository;
 use App\Validator\CompatibleAllocationType;
+use App\Validator\SingleOverflowPerScenario;
 use Doctrine\ORM\Mapping as ORM;
 use Symfony\Component\Uid\UuidV7;
 
@@ -13,6 +14,7 @@ use Symfony\Component\Uid\UuidV7;
 #[ORM\Table(name: 'bucket')]
 #[ORM\UniqueConstraint(name: 'unique_scenario_priority', columns: ['scenario_id', 'priority'])]
 #[CompatibleAllocationType]
+#[SingleOverflowPerScenario]
 class Bucket
 {
     #[ORM\Id]
diff --git a/src/Repository/BucketRepository.php b/src/Repository/BucketRepository.php
index e3794f3..16eff92 100644
--- a/src/Repository/BucketRepository.php
+++ b/src/Repository/BucketRepository.php
@@ -3,8 +3,11 @@
 namespace App\Repository;
 
 use App\Entity\Bucket;
+use App\Entity\Scenario;
+use App\Enum\BucketType;
 use Doctrine\Bundle\DoctrineBundle\Repository\ServiceEntityRepository;
 use Doctrine\Persistence\ManagerRegistry;
+use Symfony\Component\Uid\UuidV7;
 
 /**
  * @extends ServiceEntityRepository<Bucket>
@@ -16,28 +19,20 @@ class BucketRepository extends ServiceEntityRepository
         parent::__construct($registry, Bucket::class);
     }
 
-    //    /**
-    //     * @return Bucket[] Returns an array of Bucket objects
-    //     */
-    //    public function findByExampleField($value): array
-    //    {
-    //        return $this->createQueryBuilder('b')
-    //            ->andWhere('b.exampleField = :val')
-    //            ->setParameter('val', $value)
-    //            ->orderBy('b.id', 'ASC')
-    //            ->setMaxResults(10)
-    //            ->getQuery()
-    //            ->getResult()
-    //        ;
-    //    }
+    public function countOverflowBucketsForScenario(Scenario $scenario, ?UuidV7 $excludeId = null): int
+    {
+        $qb = $this->createQueryBuilder('b')
+            ->select('COUNT(b.id)')
+            ->andWhere('b.scenario = :scenario')
+            ->andWhere('b.type = :overflow')
+            ->setParameter('scenario', $scenario)
+            ->setParameter('overflow', BucketType::OVERFLOW);
 
-    //    public function findOneBySomeField($value): ?Bucket
-    //    {
-    //        return $this->createQueryBuilder('b')
-    //            ->andWhere('b.exampleField = :val')
-    //            ->setParameter('val', $value)
-    //            ->getQuery()
-    //            ->getOneOrNullResult()
-    //        ;
-    //    }
+        if (null !== $excludeId) {
+            $qb->andWhere('b.id != :excludeId')
+                ->setParameter('excludeId', $excludeId, 'uuid');
+        }
+
+        return (int) $qb->getQuery()->getSingleScalarResult();
+    }
 }
diff --git a/src/Validator/SingleOverflowPerScenario.php b/src/Validator/SingleOverflowPerScenario.php
new file mode 100644
index 0000000..605935a
--- /dev/null
+++ b/src/Validator/SingleOverflowPerScenario.php
@@ -0,0 +1,18 @@
+<?php
+
+namespace App\Validator;
+
+use Symfony\Component\Validator\Constraint;
+
+#[\Attribute(\Attribute::TARGET_CLASS)]
+final class SingleOverflowPerScenario extends Constraint
+{
+    public string $message = 'A scenario can only have one overflow bucket.';
+
+    public const string TOO_MANY_OVERFLOW_ERROR = 'TOO_MANY_OVERFLOW_ERROR';
+
+    public function getTargets(): string
+    {
+        return self::CLASS_CONSTRAINT;
+    }
+}
diff --git a/src/Validator/SingleOverflowPerScenarioValidator.php b/src/Validator/SingleOverflowPerScenarioValidator.php
new file mode 100644
index 0000000..619ab75
--- /dev/null
+++ b/src/Validator/SingleOverflowPerScenarioValidator.php
@@ -0,0 +1,44 @@
+<?php
+
+namespace App\Validator;
+
+use App\Entity\Bucket;
+use App\Enum\BucketType;
+use App\Repository\BucketRepository;
+use Symfony\Component\Validator\Constraint;
+use Symfony\Component\Validator\ConstraintValidator;
+use Symfony\Component\Validator\Exception\UnexpectedTypeException;
+
+final class SingleOverflowPerScenarioValidator extends ConstraintValidator
+{
+    public function __construct(private readonly BucketRepository $bucketRepository)
+    {
+    }
+
+    public function validate(mixed $value, Constraint $constraint): void
+    {
+        if (!$constraint instanceof SingleOverflowPerScenario) {
+            throw new UnexpectedTypeException($constraint, SingleOverflowPerScenario::class);
+        }
+
+        if (!$value instanceof Bucket) {
+            return;
+        }
+
+        if (BucketType::OVERFLOW !== $value->getType()) {
+            return;
+        }
+
+        $existing = $this->bucketRepository->countOverflowBucketsForScenario(
+            $value->getScenario(),
+            $value->getId(),
+        );
+
+        if ($existing > 0) {
+            $this->context->buildViolation($constraint->message)
+                ->atPath('type')
+                ->setCode(SingleOverflowPerScenario::TOO_MANY_OVERFLOW_ERROR)
+                ->addViolation();
+        }
+    }
+}
diff --git a/tests/Repository/BucketRepositoryTest.php b/tests/Repository/BucketRepositoryTest.php
new file mode 100644
index 0000000..6f9c2d5
--- /dev/null
+++ b/tests/Repository/BucketRepositoryTest.php
@@ -0,0 +1,125 @@
+<?php
+
+namespace App\Tests\Repository;
+
+use App\Entity\Bucket;
+use App\Entity\Scenario;
+use App\Enum\BucketAllocationType;
+use App\Enum\BucketType;
+use App\Repository\BucketRepository;
+use Doctrine\ORM\EntityManagerInterface;
+use Symfony\Bundle\FrameworkBundle\Test\KernelTestCase;
+
+final class BucketRepositoryTest extends KernelTestCase
+{
+    private EntityManagerInterface $em;
+    private BucketRepository $repository;
+
+    protected function setUp(): void
+    {
+        self::bootKernel();
+        $this->em = self::getContainer()->get(EntityManagerInterface::class);
+        $this->repository = self::getContainer()->get(BucketRepository::class);
+    }
+
+    public function testCountIsZeroWhenScenarioHasNoOverflowBucket(): void
+    {
+        $scenario = $this->persistScenario();
+
+        $this->persistBucket($scenario, BucketType::NEED, 1);
+        $this->persistBucket($scenario, BucketType::WANT, 2);
+
+        self::assertSame(
+            0,
+            $this->repository->countOverflowBucketsForScenario($scenario, null),
+            'A scenario with no OVERFLOW bucket must count zero.',
+        );
+    }
+
+    public function testCountIsOneWhenScenarioHasOneOverflowBucket(): void
+    {
+        $scenario = $this->persistScenario();
+
+        $this->persistBucket($scenario, BucketType::NEED, 1);
+        $this->persistBucket($scenario, BucketType::OVERFLOW, 2);
+
+        self::assertSame(
+            1,
+            $this->repository->countOverflowBucketsForScenario($scenario, null),
+            'A scenario with exactly one OVERFLOW bucket must count one.',
+        );
+    }
+
+    public function testCountExcludesTheGivenBucketIdFromTheTally(): void
+    {
+        $scenario = $this->persistScenario();
+
+        $overflow = $this->persistBucket($scenario, BucketType::OVERFLOW, 1);
+
+        self::assertSame(
+            0,
+            $this->repository->countOverflowBucketsForScenario($scenario, $overflow->getId()),
+            'Excluding the existing overflow bucket\'s own id must drop the count to zero — '
+            .'this is what allows updating the sole overflow bucket without falsely tripping the rule.',
+        );
+    }
+
+    public function testCountIsScopedToTheGivenScenarioOnly(): void
+    {
+        $scenarioA = $this->persistScenario('Scenario A');
+        $scenarioB = $this->persistScenario('Scenario B');
+
+        $this->persistBucket($scenarioA, BucketType::OVERFLOW, 1);
+
+        self::assertSame(
+            0,
+            $this->repository->countOverflowBucketsForScenario($scenarioB, null),
+            'An OVERFLOW bucket belonging to a different scenario must not be counted.',
+        );
+    }
+
+    public function testNeedAndWantBucketsDoNotCountTowardTheOverflowTotal(): void
+    {
+        $scenario = $this->persistScenario();
+
+        $this->persistBucket($scenario, BucketType::NEED, 1);
+        $this->persistBucket($scenario, BucketType::WANT, 2);
+
+        self::assertSame(
+            0,
+            $this->repository->countOverflowBucketsForScenario($scenario, null),
+            'NEED and WANT buckets must never count toward the overflow tally.',
+        );
+    }
+
+    private function persistScenario(string $name = 'Household Budget'): Scenario
+    {
+        $scenario = new Scenario();
+        $scenario->setName($name);
+        $this->em->persist($scenario);
+        $this->em->flush();
+
+        return $scenario;
+    }
+
+    private function persistBucket(Scenario $scenario, BucketType $type, int $priority): Bucket
+    {
+        $bucket = new Bucket();
+        $bucket->setScenario($scenario);
+        $bucket->setType($type);
+        $bucket->setName('Bucket '.$priority);
+        $bucket->setPriority($priority);
+        $bucket->setSortOrder($priority);
+        $bucket->setAllocationType(
+            BucketType::OVERFLOW === $type ? BucketAllocationType::UNLIMITED : BucketAllocationType::FIXED_LIMIT,
+        );
+        $bucket->setAllocationValue(BucketType::OVERFLOW === $type ? null : 50000);
+        $bucket->setStartingAmount(0);
+        $bucket->setBufferMultiplier('0.00');
+
+        $this->em->persist($bucket);
+        $this->em->flush();
+
+        return $bucket;
+    }
+}
diff --git a/tests/Validator/SingleOverflowPerScenarioTest.php b/tests/Validator/SingleOverflowPerScenarioTest.php
new file mode 100644
index 0000000..1d07767
--- /dev/null
+++ b/tests/Validator/SingleOverflowPerScenarioTest.php
@@ -0,0 +1,136 @@
+<?php
+
+namespace App\Tests\Validator;
+
+use App\Entity\Bucket;
+use App\Entity\Scenario;
+use App\Enum\BucketAllocationType;
+use App\Enum\BucketType;
+use App\Validator\SingleOverflowPerScenario;
+use Doctrine\ORM\EntityManagerInterface;
+use Symfony\Bundle\FrameworkBundle\Test\KernelTestCase;
+use Symfony\Component\Validator\ConstraintViolationListInterface;
+use Symfony\Component\Validator\Validator\ValidatorInterface;
+
+final class SingleOverflowPerScenarioTest extends KernelTestCase
+{
+    private EntityManagerInterface $em;
+    private ValidatorInterface $validator;
+
+    protected function setUp(): void
+    {
+        self::bootKernel();
+        $this->em = self::getContainer()->get(EntityManagerInterface::class);
+        $this->validator = self::getContainer()->get(ValidatorInterface::class);
+    }
+
+    public function testSecondOverflowBucketOnTheSameScenarioRaisesAViolation(): void
+    {
+        $scenario = new Scenario();
+        $scenario->setName('Household Budget');
+        $this->em->persist($scenario);
+        $this->em->flush();
+
+        $existingOverflow = $this->makeBucket($scenario, BucketType::OVERFLOW, 1);
+        $this->em->persist($existingOverflow);
+        $this->em->flush();
+
+        $secondOverflow = $this->makeBucket($scenario, BucketType::OVERFLOW, 2);
+
+        $violations = $this->validator->validate($secondOverflow);
+
+        $matching = $this->violationsAt($violations, 'type');
+
+        self::assertNotCount(
+            0,
+            $matching,
+            'A second OVERFLOW bucket on a scenario that already has one must raise a violation on type. '
+            .'The existing overflow bucket is persisted-but-uncommitted in this test\'s DAMA transaction — '
+            .'the validator\'s repository query must still see it within the same transaction.',
+        );
+    }
+
+    public function testTheSoleExistingOverflowBucketValidatesWithNoSingleOverflowViolation(): void
+    {
+        $scenario = new Scenario();
+        $scenario->setName('Household Budget');
+        $this->em->persist($scenario);
+        $this->em->flush();
+
+        $overflow = $this->makeBucket($scenario, BucketType::OVERFLOW, 1);
+        $this->em->persist($overflow);
+        $this->em->flush();
+
+        $violations = $this->validator->validate($overflow);
+
+        $matching = $this->violationsAt($violations, 'type');
+
+        self::assertCount(
+            0,
+            $matching,
+            'The single existing OVERFLOW bucket must not raise this constraint against itself '
+            .'(exclude-self-by-id must be working).',
+        );
+    }
+
+    public function testFirstOverflowBucketOnAScenarioWithNoOverflowBucketsIsValid(): void
+    {
+        $scenario = new Scenario();
+        $scenario->setName('Household Budget');
+        $this->em->persist($scenario);
+        $this->em->flush();
+
+        $needBucket = $this->makeBucket($scenario, BucketType::NEED, 1);
+        $this->em->persist($needBucket);
+        $this->em->flush();
+
+        $firstOverflow = $this->makeBucket($scenario, BucketType::OVERFLOW, 2);
+
+        $violations = $this->validator->validate($firstOverflow);
+
+        $matching = $this->violationsAt($violations, 'type');
+
+        self::assertCount(
+            0,
+            $matching,
+            'A brand-new, unpersisted OVERFLOW bucket on a scenario with zero existing OVERFLOW buckets '
+            .'must not raise this constraint — this is the common create-the-first-overflow-bucket flow.',
+        );
+    }
+
+    private function makeBucket(Scenario $scenario, BucketType $type, int $priority): Bucket
+    {
+        $bucket = new Bucket();
+        $bucket->setScenario($scenario);
+        $bucket->setType($type);
+        $bucket->setName('Bucket '.$priority);
+        $bucket->setPriority($priority);
+        $bucket->setSortOrder($priority);
+        $bucket->setAllocationType(
+            BucketType::OVERFLOW === $type ? BucketAllocationType::UNLIMITED : BucketAllocationType::FIXED_LIMIT,
+        );
+        $bucket->setAllocationValue(BucketType::OVERFLOW === $type ? null : 50000);
+        $bucket->setStartingAmount(0);
+        $bucket->setBufferMultiplier('0.00');
+
+        return $bucket;
+    }
+
+    /**
+     * @return list<\Symfony\Component\Validator\ConstraintViolationInterface>
+     */
+    private function violationsAt(ConstraintViolationListInterface $violations, string $propertyPath): array
+    {
+        $matching = [];
+
+        foreach ($violations as $violation) {
+            if ($violation->getPropertyPath() === $propertyPath
+                && $violation->getConstraint() instanceof SingleOverflowPerScenario
+            ) {
+                $matching[] = $violation;
+            }
+        }
+
+        return $matching;
+    }
+}
diff --git a/tests/Validator/SingleOverflowPerScenarioValidatorTest.php b/tests/Validator/SingleOverflowPerScenarioValidatorTest.php
new file mode 100644
index 0000000..191b448
--- /dev/null
+++ b/tests/Validator/SingleOverflowPerScenarioValidatorTest.php
@@ -0,0 +1,97 @@
+<?php
+
+namespace App\Tests\Validator;
+
+use App\Entity\Bucket;
+use App\Entity\Scenario;
+use App\Enum\BucketAllocationType;
+use App\Enum\BucketType;
+use App\Repository\BucketRepository;
+use App\Validator\SingleOverflowPerScenario;
+use App\Validator\SingleOverflowPerScenarioValidator;
+use PHPUnit\Framework\MockObject\Stub;
+use Symfony\Component\Validator\ConstraintValidatorInterface;
+use Symfony\Component\Validator\Test\ConstraintValidatorTestCase;
+
+/**
+ * @extends ConstraintValidatorTestCase<SingleOverflowPerScenarioValidator>
+ */
+final class SingleOverflowPerScenarioValidatorTest extends ConstraintValidatorTestCase
+{
+    private BucketRepository&Stub $bucketRepository;
+
+    protected function createValidator(): ConstraintValidatorInterface
+    {
+        $this->bucketRepository = $this->createStub(BucketRepository::class);
+
+        return new SingleOverflowPerScenarioValidator($this->bucketRepository);
+    }
+
+    public function testOverflowBucketIsValidWhenRepositoryReportsNoOtherOverflowBuckets(): void
+    {
+        $this->bucketRepository
+            ->method('countOverflowBucketsForScenario')
+            ->willReturn(0);
+
+        $bucket = $this->makeBucket(BucketType::OVERFLOW);
+
+        $this->validator->validate($bucket, new SingleOverflowPerScenario());
+
+        $this->assertNoViolation();
+    }
+
+    public function testOverflowBucketIsInvalidWhenRepositoryReportsAnExistingOverflowBucket(): void
+    {
+        $this->bucketRepository
+            ->method('countOverflowBucketsForScenario')
+            ->willReturn(1);
+
+        $bucket = $this->makeBucket(BucketType::OVERFLOW);
+
+        $this->validator->validate($bucket, new SingleOverflowPerScenario());
+
+        $this->buildViolation('A scenario can only have one overflow bucket.')
+            ->atPath('property.path.type')
+            ->setCode(SingleOverflowPerScenario::TOO_MANY_OVERFLOW_ERROR)
+            ->assertRaised();
+    }
+
+    public function testNeedBucketIsValid(): void
+    {
+        $bucket = $this->makeBucket(BucketType::NEED);
+
+        $this->validator->validate($bucket, new SingleOverflowPerScenario());
+
+        $this->assertNoViolation();
+    }
+
+    public function testWantBucketIsValid(): void
+    {
+        $bucket = $this->makeBucket(BucketType::WANT);
+
+        $this->validator->validate($bucket, new SingleOverflowPerScenario());
+
+        $this->assertNoViolation();
+    }
+
+    private function makeBucket(BucketType $type): Bucket
+    {
+        $scenario = new Scenario();
+        $scenario->setName('Household Budget');
+
+        $bucket = new Bucket();
+        $bucket->setScenario($scenario);
+        $bucket->setType($type);
+        $bucket->setName('Test Bucket');
+        $bucket->setPriority(1);
+        $bucket->setSortOrder(0);
+        $bucket->setAllocationType(
+            BucketType::OVERFLOW === $type ? BucketAllocationType::UNLIMITED : BucketAllocationType::FIXED_LIMIT,
+        );
+        $bucket->setAllocationValue(BucketType::OVERFLOW === $type ? null : 50000);
+        $bucket->setStartingAmount(0);
+        $bucket->setBufferMultiplier('0.00');
+
+        return $bucket;
+    }
+}