diff --git a/tests/services/s3Service.test.ts b/tests/services/s3Service.test.ts
new file mode 100644
index 0000000..6a3915b
--- /dev/null
+++ b/tests/services/s3Service.test.ts
@@ -0,0 +1,118 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+
+vi.mock('@aws-sdk/s3-request-presigner');
+
+vi.mock('@aws-sdk/client-s3', async (importOriginal) => {
+    const originalModule = await importOriginal<typeof import('@aws-sdk/client-s3')>();
+    return {
+        ...originalModule,
+        S3Client: vi.fn(),
+    };
+});
+
+import { S3Service, S3ClientConfig } from '../../src/services/s3Service';
+import { S3Client, CreateBucketCommand, PutObjectCommand, GetObjectCommand } from '@aws-sdk/client-s3';
+import { getSignedUrl } from '@aws-sdk/s3-request-presigner';
+
+const testConfig: S3ClientConfig = {
+    endpoint: 'http://test-minio',
+    port: 9000,
+    accessKey: 'test-key',
+    secretAccessKey: 'test-secret',
+    bucket: 'test-bucket',
+};
+
+const MockS3Client = vi.mocked(S3Client);
+const mockSend = vi.fn(); // Das ist unser gefälschter "send"-Befehl
+const mockGetSignedUrl = vi.mocked(getSignedUrl);
+
+describe('S3Service', () => {
+    let s3Service: S3Service;
+
+    beforeEach(() => {
+        vi.clearAllMocks();
+
+        // @ts-ignore
+        S3Service.instance = undefined;
+
+        MockS3Client.mockImplementation(() => ({
+            send: mockSend,
+        }) as any);
+
+        s3Service = S3Service.getInstance(testConfig);
+    });
+
+    describe('Initialization and Bucket Creation', () => {
+        it('should create a singleton instance correctly', () => {
+            const instance1 = S3Service.getInstance(testConfig);
+            const instance2 = S3Service.getInstance(); // Ohne Config, da schon initialisiert
+            expect(instance1).toBe(instance2);
+            expect(MockS3Client).toHaveBeenCalledOnce();
+        });
+
+        it('should call ensureBucketExists and handle existing buckets gracefully', async () => {
+            mockSend.mockRejectedValue({ name: 'BucketAlreadyOwnedByYou' });
+
+            await expect(s3Service.ensureBucketExists()).resolves.toBeUndefined();
+
+            expect(mockSend).toHaveBeenCalledWith(expect.any(CreateBucketCommand));
+        });
+
+        it('should create a new bucket if it does not exist', async () => {
+            mockSend.mockResolvedValue({});
+
+            await expect(s3Service.ensureBucketExists()).resolves.toBeUndefined();
+
+            expect(mockSend).toHaveBeenCalledWith(expect.any(CreateBucketCommand));
+        });
+    });
+
+    describe('uploadFile', () => {
+        it('should upload a file and return the correct object key', async () => {
+            const mockFile = {
+                originalname: 'test-image.jpg',
+                buffer: Buffer.from('test-data'),
+                mimetype: 'image/jpeg',
+            };
+
+            const userId = 'user-123';
+
+            const objectKey = await s3Service.uploadFile(mockFile, userId);
+
+            expect(objectKey).toMatch(/^user-user-123\/[a-f0-9-]+\.jpg$/);
+
+            expect(mockSend).toHaveBeenCalledOnce();
+            expect(mockSend).toHaveBeenCalledWith(expect.any(PutObjectCommand));
+
+            const sentCommand = (mockSend.mock.calls[0][0] as PutObjectCommand).input;
+
+            expect(sentCommand.Bucket).toBe('test-bucket');
+            expect(sentCommand.Key).toBe(objectKey);
+            expect(sentCommand.Body).toBe(mockFile.buffer);
+        });
+    });
+
+    describe('getSignedDownloadUrl', () => {
+        it('should generate a signed URL for a given object key', async () => {
+            const objectKey = 'user-123/image.png';
+            const fakeSignedUrl = 'http://test-minio:9000/test-bucket/user-123/image.png?signed=true';
+
+            mockGetSignedUrl.mockResolvedValue(fakeSignedUrl);
+
+            const signedUrl = await s3Service.getSignedDownloadUrl(objectKey, 300);
+
+            expect(signedUrl).toBe(fakeSignedUrl);
+
+            expect(mockGetSignedUrl).toHaveBeenCalledOnce();
+            expect(mockGetSignedUrl).toHaveBeenCalledWith(
+                expect.any(Object),
+                expect.any(GetObjectCommand),
+                { expiresIn: 300 }
+            );
+
+            const passedCommand = (mockGetSignedUrl.mock.calls[0][1] as GetObjectCommand).input;
+            expect(passedCommand.Bucket).toBe('test-bucket');
+            expect(passedCommand.Key).toBe(objectKey);
+        });
+    });
+});
\ No newline at end of file