diff --git a/src/State/EmployeeRttPaymentProcessor.php b/src/State/EmployeeRttPaymentProcessor.php
index 4b661ab..48d5e75 100644
--- a/src/State/EmployeeRttPaymentProcessor.php
+++ b/src/State/EmployeeRttPaymentProcessor.php
@@ -12,8 +12,10 @@ use App\Entity\EmployeeRttPayment;
 use App\Repository\EmployeeRepository;
 use App\Repository\EmployeeRttPaymentRepository;
 use App\Service\AuditLogger;
+use App\Service\Contracts\EmployeeContractPhaseResolver;
 use DateTimeImmutable;
 use Doctrine\ORM\EntityManagerInterface;
+use Psr\Clock\ClockInterface;
 use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
 use Symfony\Component\HttpKernel\Exception\UnprocessableEntityHttpException;
 
@@ -24,6 +26,8 @@ final readonly class EmployeeRttPaymentProcessor implements ProcessorInterface
         private EmployeeRttPaymentRepository $rttPaymentRepository,
         private EntityManagerInterface $entityManager,
         private AuditLogger $auditLogger,
+        private EmployeeContractPhaseResolver $phaseResolver,
+        private ClockInterface $clock,
     ) {}
 
     public function process(mixed $data, Operation $operation, array $uriVariables = [], array $context = []): EmployeeRttPaymentInput
@@ -48,6 +52,8 @@ final readonly class EmployeeRttPaymentProcessor implements ProcessorInterface
 
         $year = $data->year ?? $this->resolveCurrentExerciseYear();
 
+        $this->assertYearAllowedForPayment($employee, $year);
+
         $payment = $this->rttPaymentRepository->findOneByEmployeeYearMonth($employee, $year, $data->month);
 
         if (null === $payment) {
@@ -83,10 +89,44 @@ final readonly class EmployeeRttPaymentProcessor implements ProcessorInterface
 
     private function resolveCurrentExerciseYear(): int
     {
-        $today = new DateTimeImmutable('today');
-        $year  = (int) $today->format('Y');
-        $month = (int) $today->format('n');
+        return $this->resolveExerciseYearForDate($this->clock->now());
+    }
+
+    /**
+     * Map a date to the RTT exercise year it belongs to (Juin N-1 → Mai N convention).
+     */
+    private function resolveExerciseYearForDate(DateTimeImmutable $date): int
+    {
+        $year  = (int) $date->format('Y');
+        $month = (int) $date->format('n');
 
         return $month >= 6 ? $year + 1 : $year;
     }
+
+    /**
+     * Allow payment when the requested exercise is either the current one
+     * or the last exercise of a closed contract phase (the one containing
+     * the phase end date). Reject any other exercise (past or future).
+     */
+    private function assertYearAllowedForPayment(Employee $employee, int $year): void
+    {
+        $currentExerciseYear = $this->resolveCurrentExerciseYear();
+        if ($year === $currentExerciseYear) {
+            return;
+        }
+
+        $phases = $this->phaseResolver->resolvePhases($employee);
+        foreach ($phases as $phase) {
+            if ($phase->isCurrent || null === $phase->endDate) {
+                continue;
+            }
+            if ($year === $this->resolveExerciseYearForDate($phase->endDate)) {
+                return;
+            }
+        }
+
+        throw new UnprocessableEntityHttpException(
+            'RTT payment is only allowed on the current exercise or the last exercise of a closed contract phase.'
+        );
+    }
 }
diff --git a/tests/State/EmployeeRttPaymentProcessorTest.php b/tests/State/EmployeeRttPaymentProcessorTest.php
new file mode 100644
index 0000000..87e877f
--- /dev/null
+++ b/tests/State/EmployeeRttPaymentProcessorTest.php
@@ -0,0 +1,167 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Tests\State;
+
+use App\Entity\Contract;
+use App\Entity\Employee;
+use App\Entity\EmployeeContractPeriod;
+use App\Enum\ContractNature;
+use App\Enum\TrackingMode;
+use App\Service\Contracts\EmployeeContractPhaseResolver;
+use App\State\EmployeeRttPaymentProcessor;
+use DateTimeImmutable;
+use PHPUnit\Framework\TestCase;
+use Psr\Clock\ClockInterface;
+use ReflectionClass;
+use ReflectionProperty;
+use Symfony\Component\HttpKernel\Exception\UnprocessableEntityHttpException;
+
+/**
+ * @internal
+ *
+ * Exercises the year-acceptance guard of EmployeeRttPaymentProcessor.
+ *
+ * The processor depends on final repositories (EmployeeRepository,
+ * EmployeeRttPaymentRepository) which PHPUnit cannot double. The guard logic
+ * lives in a private helper (assertYearAllowedForPayment) tested directly via
+ * reflection — same pattern used in EmployeeRttSummaryProviderTest.
+ */
+final class EmployeeRttPaymentProcessorTest extends TestCase
+{
+    public function testPaymentAllowedOnCurrentExercise(): void
+    {
+        // Today = 2026-05-19 (env clock) → current exercise = 2026.
+        $employee  = $this->buildEmployeeWithTransition('2020-06-01', '2026-04-30', '2026-05-01');
+        $processor = $this->buildProcessorWithClock(new DateTimeImmutable('2026-05-19'));
+
+        $this->invokePrivate($processor, 'assertYearAllowedForPayment', $employee, 2026);
+
+        // No exception → guard accepts current exercise.
+        self::assertTrue(true);
+    }
+
+    public function testPaymentAllowedOnLastExerciseOfClosedPhase(): void
+    {
+        // Phase 39h closed 2026-04-30, FORFAIT from 2026-05-01.
+        // Exercise 2026 (Juin 2025 → Mai 2026) contains the H39 phase end date.
+        // Payment must be allowed on exercise 2026 even when current exercise is 2027.
+        $employee  = $this->buildEmployeeWithTransition('2020-06-01', '2026-04-30', '2026-05-01');
+        $processor = $this->buildProcessorWithClock(new DateTimeImmutable('2027-01-15'));
+
+        $this->invokePrivate($processor, 'assertYearAllowedForPayment', $employee, 2026);
+
+        self::assertTrue(true);
+    }
+
+    public function testPaymentRejectedOnEarlierExerciseOfClosedPhase(): void
+    {
+        $employee  = $this->buildEmployeeWithTransition('2020-06-01', '2026-04-30', '2026-05-01');
+        $processor = $this->buildProcessorWithClock(new DateTimeImmutable('2027-01-15'));
+
+        $this->expectException(UnprocessableEntityHttpException::class);
+        $this->invokePrivate($processor, 'assertYearAllowedForPayment', $employee, 2024);
+    }
+
+    public function testPaymentRejectedOnFutureExercise(): void
+    {
+        $employee  = $this->buildEmployeeWithTransition('2020-06-01', '2026-04-30', '2026-05-01');
+        $processor = $this->buildProcessorWithClock(new DateTimeImmutable('2026-05-19'));
+
+        $this->expectException(UnprocessableEntityHttpException::class);
+        $this->invokePrivate($processor, 'assertYearAllowedForPayment', $employee, 2030);
+    }
+
+    // -----------------------------------------------------------------------
+    // Test harness helpers.
+    // -----------------------------------------------------------------------
+
+    /**
+     * Build a two-period employee transitioning from H39 to FORFAIT.
+     */
+    private function buildEmployeeWithTransition(string $h39Start, string $h39End, string $forfaitStart): Employee
+    {
+        $employee = new Employee();
+        $this->setEntityId($employee, 1);
+
+        $h39Contract = new Contract();
+        $h39Contract->setName('39H');
+        $h39Contract->setTrackingMode(TrackingMode::TIME->value);
+        $h39Contract->setWeeklyHours(39);
+
+        $forfaitContract = new Contract();
+        $forfaitContract->setName('Forfait');
+        $forfaitContract->setTrackingMode(TrackingMode::PRESENCE->value);
+        $forfaitContract->setWeeklyHours(null);
+
+        $h39Period = new EmployeeContractPeriod();
+        $this->setEntityId($h39Period, 1);
+        $h39Period->setEmployee($employee);
+        $h39Period->setContract($h39Contract);
+        $h39Period->setStartDate(new DateTimeImmutable($h39Start));
+        $h39Period->setEndDate(new DateTimeImmutable($h39End));
+        $h39Period->setContractNature(ContractNature::CDI);
+        $h39Period->setIsDriver(false);
+
+        $forfaitPeriod = new EmployeeContractPeriod();
+        $this->setEntityId($forfaitPeriod, 2);
+        $forfaitPeriod->setEmployee($employee);
+        $forfaitPeriod->setContract($forfaitContract);
+        $forfaitPeriod->setStartDate(new DateTimeImmutable($forfaitStart));
+        $forfaitPeriod->setEndDate(null);
+        $forfaitPeriod->setContractNature(ContractNature::CDI);
+        $forfaitPeriod->setIsDriver(false);
+
+        $employee->getContractPeriods()->add($h39Period);
+        $employee->getContractPeriods()->add($forfaitPeriod);
+
+        return $employee;
+    }
+
+    /**
+     * Build an uninitialized processor with a fixed clock. The repositories are
+     * declared on final classes that PHPUnit cannot double, so we bypass full
+     * instantiation via newInstanceWithoutConstructor and only seed the
+     * properties the tested private guard reads: phaseResolver + clock.
+     */
+    private function buildProcessorWithClock(DateTimeImmutable $today): EmployeeRttPaymentProcessor
+    {
+        $reflection = new ReflectionClass(EmployeeRttPaymentProcessor::class);
+        $processor  = $reflection->newInstanceWithoutConstructor();
+
+        $clock = new readonly class($today) implements ClockInterface {
+            public function __construct(private DateTimeImmutable $now) {}
+
+            public function now(): DateTimeImmutable
+            {
+                return $this->now;
+            }
+        };
+
+        $this->setReadonlyProperty($processor, 'phaseResolver', new EmployeeContractPhaseResolver());
+        $this->setReadonlyProperty($processor, 'clock', $clock);
+
+        return $processor;
+    }
+
+    private function invokePrivate(object $obj, string $method, mixed ...$args): mixed
+    {
+        $reflection = new ReflectionClass($obj::class);
+        $m          = $reflection->getMethod($method);
+
+        return $m->invoke($obj, ...$args);
+    }
+
+    private function setReadonlyProperty(object $obj, string $property, mixed $value): void
+    {
+        $reflection = new ReflectionProperty($obj::class, $property);
+        $reflection->setValue($obj, $value);
+    }
+
+    private function setEntityId(object $entity, int $id): void
+    {
+        $reflection = new ReflectionProperty($entity::class, 'id');
+        $reflection->setValue($entity, $id);
+    }
+}