diff --git a/apps/backend/src/__tests__/cards.test.ts b/apps/backend/src/__tests__/cards.test.ts
index 3542a539..1fcb5302 100644
--- a/apps/backend/src/__tests__/cards.test.ts
+++ b/apps/backend/src/__tests__/cards.test.ts
@@ -305,6 +305,117 @@ describe('PUT /api/cards/:id — link ownership validation', () => {
   });
 });
 
+// ─────────────────────────────────────────────────────────────────────────────
+// PUT /api/cards/:id/default — serialization & retry behaviour
+// ─────────────────────────────────────────────────────────────────────────────
+
+describe('PUT /api/cards/:id/default — serialization isolation & retry', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it('passes isolationLevel Serializable to $transaction', async () => {
+    mockPrisma.card.findFirst.mockResolvedValue(mockCard);
+    mockPrisma.$transaction.mockImplementation(
+      async (callback: (tx: typeof mockPrisma) => Promise<unknown>, options?: unknown) => {
+        expect(options).toEqual({ isolationLevel: 'Serializable' });
+        mockPrisma.card.updateMany.mockResolvedValue({ count: 2 });
+        mockPrisma.card.update.mockResolvedValue({ ...mockCard, isDefault: true });
+        return callback(mockPrisma);
+      },
+    );
+
+    const app = await buildApp();
+    const res = await app.inject({ method: 'PUT', url: `/api/cards/${CARD_ID}/default` });
+
+    expect(res.statusCode).toBe(200);
+    expect(mockPrisma.$transaction).toHaveBeenCalledOnce();
+  });
+
+  it('retries on P2034 (serialization conflict) and succeeds on second attempt', async () => {
+    mockPrisma.card.findFirst.mockResolvedValue(mockCard);
+
+    const p2034 = Object.assign(new Error('Serialization failure'), { code: 'P2034' });
+    let callCount = 0;
+
+    mockPrisma.$transaction.mockImplementation(
+      async (callback: (tx: typeof mockPrisma) => Promise<unknown>) => {
+        callCount++;
+        if (callCount === 1) { throw p2034; }
+        mockPrisma.card.updateMany.mockResolvedValue({ count: 2 });
+        mockPrisma.card.update.mockResolvedValue({ ...mockCard, isDefault: true });
+        return callback(mockPrisma);
+      },
+    );
+
+    const app = await buildApp();
+    const res = await app.inject({ method: 'PUT', url: `/api/cards/${CARD_ID}/default` });
+
+    expect(res.statusCode).toBe(200);
+    expect(res.json().message).toBe('Default card updated');
+    expect(mockPrisma.$transaction).toHaveBeenCalledTimes(2);
+  });
+
+  it('returns 500 after exhausting all 3 retry attempts on persistent P2034', async () => {
+    mockPrisma.card.findFirst.mockResolvedValue(mockCard);
+
+    const p2034 = Object.assign(new Error('Serialization failure'), { code: 'P2034' });
+    mockPrisma.$transaction.mockRejectedValue(p2034);
+
+    const app = await buildApp();
+    const res = await app.inject({ method: 'PUT', url: `/api/cards/${CARD_ID}/default` });
+
+    expect(res.statusCode).toBe(500);
+    expect(mockPrisma.$transaction).toHaveBeenCalledTimes(3);
+  });
+
+  it('does not retry on non-P2034 errors and returns 500 immediately', async () => {
+    mockPrisma.card.findFirst.mockResolvedValue(mockCard);
+
+    const dbError = new Error('Connection lost');
+    mockPrisma.$transaction.mockRejectedValue(dbError);
+
+    const app = await buildApp();
+    const res = await app.inject({ method: 'PUT', url: `/api/cards/${CARD_ID}/default` });
+
+    expect(res.statusCode).toBe(500);
+    // Must not have retried — only one attempt for non-serialization errors
+    expect(mockPrisma.$transaction).toHaveBeenCalledTimes(1);
+  });
+
+  it('concurrent calls: the last committed transaction determines the sole default', async () => {
+    mockPrisma.card.findFirst.mockResolvedValue(mockCard);
+
+    let callCount = 0;
+    mockPrisma.$transaction.mockImplementation(
+      async (callback: (tx: typeof mockPrisma) => Promise<unknown>) => {
+        callCount++;
+        // First concurrent attempt fails (simulates DB-level serialization abort)
+        if (callCount === 1) {
+          throw Object.assign(new Error('Serialization failure'), { code: 'P2034' });
+        }
+        mockPrisma.card.updateMany.mockResolvedValue({ count: 2 });
+        mockPrisma.card.update.mockResolvedValue({ ...mockCard, isDefault: true });
+        return callback(mockPrisma);
+      },
+    );
+
+    const app = await buildApp();
+
+    // Fire both requests concurrently
+    const [res1, res2] = await Promise.all([
+      app.inject({ method: 'PUT', url: `/api/cards/${CARD_ID}/default` }),
+      app.inject({ method: 'PUT', url: `/api/cards/${CARD_ID}/default` }),
+    ]);
+
+    // Both callers ultimately succeed (one retried)
+    expect([res1.statusCode, res2.statusCode]).toEqual([200, 200]);
+    // Combined, the transaction was attempted 3 times (1 fail + 1 succeed for
+    // the first caller, 1 succeed for the second — ordering may vary)
+    expect(mockPrisma.$transaction.mock.calls.length).toBeGreaterThanOrEqual(2);
+  });
+});
+
 // ─────────────────────────────────────────────────────────────────────────────
 // DELETE /api/cards/:id
 // ─────────────────────────────────────────────────────────────────────────────
diff --git a/apps/backend/src/services/cardService.ts b/apps/backend/src/services/cardService.ts
index fd3b9903..786baaf7 100644
--- a/apps/backend/src/services/cardService.ts
+++ b/apps/backend/src/services/cardService.ts
@@ -165,10 +165,30 @@ export async function setDefaultCard(app: FastifyInstance, userId: string, id: s
     return null;
   }
 
-  await app.prisma.$transaction(async (tx: Prisma.TransactionClient) => {
-    await tx.card.updateMany({ where: { userId }, data: { isDefault: false } });
-    await tx.card.update({ where: { id }, data: { isDefault: true } });
-  });
+  const maxRetries = 3;
+  for (let attempt = 1; attempt <= maxRetries; attempt++) {
+    try {
+      await app.prisma.$transaction(
+        async (tx: Prisma.TransactionClient) => {
+          await tx.card.updateMany({ where: { userId }, data: { isDefault: false } });
+          await tx.card.update({ where: { id }, data: { isDefault: true } });
+        },
+        { isolationLevel: 'Serializable' },
+      );
+
+      return { message: 'Default card updated' };
+    } catch (error: unknown) {
+      if (
+        typeof error === 'object' && error !== null && 'code' in error &&
+        (error as { code: string }).code === 'P2034' &&
+        attempt < maxRetries ) {
+        continue;
+      }
+
+      app.log.error(error);
+      throw error;
+    }
+  }
 
-  return { message: 'Default card updated' };
+  throw new Error('Failed to set default card after retrying serialization conflicts');
 }