diff --git a/backend/tests/Unit/Comment/UseCases/CreateCommentTest.php b/backend/tests/Unit/Comment/UseCases/CreateCommentTest.php
new file mode 100644
index 0000000..f35f8e8
--- /dev/null
+++ b/backend/tests/Unit/Comment/UseCases/CreateCommentTest.php
@@ -0,0 +1,138 @@
+<?php
+
+namespace Tests\Unit\Comment\UseCases;
+
+use App\Comment\UseCases\CreateComment\CreateComment;
+use App\Comment\UseCases\CreateComment\CreateCommentRequest;
+use App\Exceptions\BadRequestException;
+use App\Post\CreatePostDto;
+use DateTimeImmutable;
+use DateTimeZone;
+use DomainException;
+use Tests\Fakes\FakeClock;
+use Tests\Fakes\FakeCommentRepository;
+use Tests\Fakes\FakePostRepository;
+use Tests\TestCase;
+
+class CreateCommentTest extends TestCase
+{
+    private FakeCommentRepository $commentRepo;
+
+    private FakePostRepository $postRepo;
+
+    private FakeClock $clock;
+
+    private DateTimeImmutable $now;
+
+    private CreateComment $useCase;
+
+    protected function setUp(): void
+    {
+        $this->now = new DateTimeImmutable(
+            '2026-05-06T12:00:00',
+            new DateTimeZone('UTC'),
+        );
+        $this->commentRepo = new FakeCommentRepository;
+        $this->postRepo = new FakePostRepository;
+        $this->clock = new FakeClock($this->now);
+        $this->useCase = new CreateComment(
+            $this->commentRepo,
+            $this->postRepo,
+            $this->clock,
+        );
+    }
+
+    private function seedPost(): int
+    {
+        $post = $this->postRepo->create(new CreatePostDto(
+            userId: 1,
+            title: 'Some Post',
+            body: 'Body.',
+            createdAt: $this->now,
+        ));
+
+        return $post->getId();
+    }
+
+    public function test_zero_post_id_throws_bad_request(): void
+    {
+        $this->expectException(BadRequestException::class);
+        $this->useCase->execute(new CreateCommentRequest(
+            postId: 0,
+            userId: 1,
+            body: 'hi',
+        ));
+    }
+
+    public function test_zero_user_id_throws_bad_request(): void
+    {
+        $postId = $this->seedPost();
+        $this->expectException(BadRequestException::class);
+        $this->useCase->execute(new CreateCommentRequest(
+            postId: $postId,
+            userId: 0,
+            body: 'hi',
+        ));
+    }
+
+    public function test_null_body_throws_bad_request(): void
+    {
+        $postId = $this->seedPost();
+        $this->expectException(BadRequestException::class);
+        $this->useCase->execute(new CreateCommentRequest(
+            postId: $postId,
+            userId: 1,
+            body: null,
+        ));
+    }
+
+    public function test_blank_body_throws_bad_request(): void
+    {
+        $postId = $this->seedPost();
+        $this->expectException(BadRequestException::class);
+        $this->useCase->execute(new CreateCommentRequest(
+            postId: $postId,
+            userId: 1,
+            body: '   ',
+        ));
+    }
+
+    public function test_unknown_post_throws_domain_exception(): void
+    {
+        $this->expectException(DomainException::class);
+        $this->useCase->execute(new CreateCommentRequest(
+            postId: 999,
+            userId: 1,
+            body: 'hi',
+        ));
+    }
+
+    public function test_valid_create_returns_comment(): void
+    {
+        $postId = $this->seedPost();
+        $created = $this->useCase->execute(new CreateCommentRequest(
+            postId: $postId,
+            userId: 5,
+            body: '  Hello world  ',
+        ));
+
+        $this->assertSame($postId, $created->getPostId());
+        $this->assertSame(5, $created->getUserId());
+        $this->assertSame('Hello world', $created->getBody());
+        $this->assertEquals($this->now, $created->getCreatedAt());
+    }
+
+    public function test_created_comment_is_findable(): void
+    {
+        $postId = $this->seedPost();
+        $created = $this->useCase->execute(new CreateCommentRequest(
+            postId: $postId,
+            userId: 5,
+            body: 'Hello',
+        ));
+
+        $found = $this->commentRepo->find($created->getId());
+        $this->assertNotNull($found);
+        $this->assertSame('Hello', $found->getBody());
+    }
+}