diff --git a/src/Dto/Matchfunding/MatchCallSubmissionCreationDto.php b/src/Dto/Matchfunding/MatchCallSubmissionCreationDto.php
index 64827d8..9299538 100644
--- a/src/Dto/Matchfunding/MatchCallSubmissionCreationDto.php
+++ b/src/Dto/Matchfunding/MatchCallSubmissionCreationDto.php
@@ -4,8 +4,10 @@ namespace App\Dto\Matchfunding;
 
 use App\ApiResource\Matchfunding\MatchCallApiResource;
 use App\ApiResource\Project\ProjectApiResource;
+use App\Validator\SingleProjectSubmissionPerCall;
 use Symfony\Component\Validator\Constraints as Assert;
 
+#[SingleProjectSubmissionPerCall()]
 class MatchCallSubmissionCreationDto
 {
     /**
diff --git a/src/Validator/SingleProjectSubmissionPerCall.php b/src/Validator/SingleProjectSubmissionPerCall.php
new file mode 100644
index 0000000..573374a
--- /dev/null
+++ b/src/Validator/SingleProjectSubmissionPerCall.php
@@ -0,0 +1,24 @@
+<?php
+
+namespace App\Validator;
+
+use Symfony\Component\Validator\Constraint;
+
+#[\Attribute]
+final class SingleProjectSubmissionPerCall extends Constraint
+{
+    public string $message = 'There is an existing MatchCallSubmission for that Project to that MatchCall.';
+
+    public function __construct(
+        public string $mode = 'strict',
+        ?array $groups = null,
+        mixed $payload = null
+    ) {
+        parent::__construct([], $groups, $payload);
+    }
+
+    public function getTargets(): string
+    {
+        return self::CLASS_CONSTRAINT;
+    }
+}
diff --git a/src/Validator/SingleProjectSubmissionPerCallValidator.php b/src/Validator/SingleProjectSubmissionPerCallValidator.php
new file mode 100644
index 0000000..4dde0a4
--- /dev/null
+++ b/src/Validator/SingleProjectSubmissionPerCallValidator.php
@@ -0,0 +1,34 @@
+<?php
+
+namespace App\Validator;
+
+use App\Dto\Matchfunding\MatchCallSubmissionCreationDto;
+use App\Repository\Matchfunding\MatchCallSubmissionRepository;
+use Symfony\Component\Validator\Constraint;
+use Symfony\Component\Validator\ConstraintValidator;
+
+final class SingleProjectSubmissionPerCallValidator extends ConstraintValidator
+{
+    public function __construct(
+        private MatchCallSubmissionRepository $matchCallSubmissionRepository
+    ) {}
+
+    /**
+     * @param MatchCallSubmissionCreationDto $value
+     */
+    public function validate(mixed $value, Constraint $constraint): void
+    {
+        $submission = $this->matchCallSubmissionRepository->findOneBy([
+            'call' => $value->call->id,
+            'project' => $value->project->id
+        ]);
+
+        if ($submission === null) {
+            return;
+        }
+
+        $this->context->buildViolation($constraint->message)
+            ->addViolation()
+        ;
+    }
+}