diff --git a/src/modules/rewards/reward.service.ts b/src/modules/rewards/reward.service.ts
index 63e852e..0602abe 100644
--- a/src/modules/rewards/reward.service.ts
+++ b/src/modules/rewards/reward.service.ts
@@ -80,17 +80,19 @@ export async function processRewardClaim(
     throw err;
   }
 
-  await db
-    .update(quizSubmissions)
-    .set({ rewardClaimed: true, txHash })
-    .where(eq(quizSubmissions.id, submissionId));
-
-  await db
-    .update(users)
-    .set({
-      credits: sql`${users.credits} + ${REWARD_AMOUNT}`,
-    })
-    .where(eq(users.id, userId));
+  await db.transaction(async (tx) => {
+    await tx
+      .update(quizSubmissions)
+      .set({ rewardClaimed: true, txHash })
+      .where(eq(quizSubmissions.id, submissionId));
+
+    await tx
+      .update(users)
+      .set({
+        credits: sql`${users.credits} + ${REWARD_AMOUNT}`,
+      })
+      .where(eq(users.id, userId));
+  });
 
   return true;
 }
diff --git a/tests/unit/services/process-reward-claim.test.ts b/tests/unit/services/process-reward-claim.test.ts
new file mode 100644
index 0000000..86078af
--- /dev/null
+++ b/tests/unit/services/process-reward-claim.test.ts
@@ -0,0 +1,227 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+
+vi.mock("@stellar/stellar-sdk", () => ({
+  default: {
+    Address: {
+      fromString: vi.fn().mockReturnValue({
+        toScVal: vi.fn().mockReturnValue("mock-sc-val"),
+      }),
+    },
+    nativeToScVal: vi.fn().mockReturnValue("mock-native-val"),
+  },
+}));
+
+vi.mock("../../../src/config/database.js", () => {
+  const mockDb = {
+    select: vi.fn(),
+    from: vi.fn(),
+    where: vi.fn(),
+    update: vi.fn(),
+    insert: vi.fn(),
+    transaction: vi.fn(),
+  };
+  return { db: mockDb };
+});
+
+vi.mock("../../../src/stellar/transactions.js", () => ({
+  invokeContract: vi.fn().mockResolvedValue("tx-hash-123"),
+}));
+
+vi.mock("../../../src/stellar/signatures.js", () => ({
+  createQuizProof: vi.fn().mockReturnValue({ signature: "base64sig" }),
+}));
+
+vi.mock("../../../src/stellar/resilience.js", () => ({
+  isCircuitBreakerError: vi.fn().mockReturnValue(false),
+}));
+
+vi.mock("../../../src/config/index.js", () => ({
+  config: {
+    STELLAR_REWARD_CONTRACT_ID: "test-reward-contract",
+  },
+}));
+
+vi.mock("../../../src/utils/logger.js", () => ({
+  logger: { info: vi.fn(), error: vi.fn(), warn: vi.fn() },
+}));
+
+vi.mock("../../../src/services/retry-queue.js", () => ({
+  enqueueReward: vi.fn().mockResolvedValue(undefined),
+}));
+
+vi.mock("../../../src/audit/index.js", () => ({
+  auditLog: vi.fn(),
+}));
+
+vi.mock("../../../src/metrics/index.js", () => ({
+  rewardClaimsTotal: { inc: vi.fn() },
+  stellarTxDurationSeconds: { observe: vi.fn() },
+}));
+
+import { db } from "../../../src/config/database.js";
+import { processRewardClaim } from "../../../src/modules/rewards/reward.service.js";
+import { invokeContract } from "../../../src/stellar/transactions.js";
+
+const mockDb = vi.mocked(db);
+
+function makeThenable(result: any[]) {
+  const obj: any = {};
+  obj.then = (resolve: Function, reject: Function) =>
+    Promise.resolve(result).then(resolve, reject);
+  obj.select = vi.fn().mockReturnValue(obj);
+  obj.from = vi.fn().mockReturnValue(obj);
+  obj.where = vi.fn().mockReturnValue(obj);
+  obj.update = vi.fn().mockReturnValue(obj);
+  obj.set = vi.fn().mockReturnValue(obj);
+  return obj;
+}
+
+describe("processRewardClaim", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("should return true when submission does not exist", async () => {
+    const submissionChain = makeThenable([]);
+
+    mockDb.select.mockReturnValue(submissionChain);
+
+    const result = await processRewardClaim("sub-1", "user-1", 5);
+    expect(result).toBe(true);
+  });
+
+  it("should return true when reward is already claimed", async () => {
+    const submissionChain = makeThenable([
+      {
+        id: "sub-1",
+        userId: "user-1",
+        score: 5,
+        rewardClaimed: true,
+        quizId: "quiz-1",
+      },
+    ]);
+
+    mockDb.select.mockReturnValue(submissionChain);
+
+    const result = await processRewardClaim("sub-1", "user-1", 5);
+    expect(result).toBe(true);
+  });
+
+  it("should return true when quiz does not exist", async () => {
+    const submissionChain = makeThenable([
+      {
+        id: "sub-1",
+        userId: "user-1",
+        score: 5,
+        rewardClaimed: false,
+        quizId: "quiz-1",
+      },
+    ]);
+    const quizChain = makeThenable([]);
+
+    mockDb.select
+      .mockReturnValueOnce(submissionChain)
+      .mockReturnValueOnce(quizChain);
+
+    const result = await processRewardClaim("sub-1", "user-1", 5);
+    expect(result).toBe(true);
+  });
+
+  it("should return true when user does not exist", async () => {
+    const submissionChain = makeThenable([
+      {
+        id: "sub-1",
+        userId: "user-1",
+        score: 5,
+        rewardClaimed: false,
+        quizId: "quiz-1",
+      },
+    ]);
+    const quizChain = makeThenable([{ id: "quiz-1", courseId: "course-1" }]);
+    const userChain = makeThenable([]);
+
+    mockDb.select
+      .mockReturnValueOnce(submissionChain)
+      .mockReturnValueOnce(quizChain)
+      .mockReturnValueOnce(userChain);
+
+    const result = await processRewardClaim("sub-1", "user-1", 5);
+    expect(result).toBe(true);
+  });
+
+  it("should successfully process claim and update DB in transaction", async () => {
+    const submissionChain = makeThenable([
+      {
+        id: "sub-1",
+        userId: "user-1",
+        score: 5,
+        rewardClaimed: false,
+        quizId: "quiz-1",
+      },
+    ]);
+    const quizChain = makeThenable([{ id: "quiz-1", courseId: "course-1" }]);
+    const userChain = makeThenable([
+      {
+        id: "user-1",
+        stellarAddress:
+          "GALICE0000000000000000000000000000000000000000000000000000000",
+      },
+    ]);
+
+    mockDb.select
+      .mockReturnValueOnce(submissionChain)
+      .mockReturnValueOnce(quizChain)
+      .mockReturnValueOnce(userChain);
+
+    mockDb.transaction.mockImplementation(async (fn: Function) => {
+      const tx: any = {};
+      tx.update = vi.fn().mockReturnValue({
+        set: vi.fn().mockReturnValue({
+          where: vi.fn().mockResolvedValue(undefined),
+        }),
+      });
+      return fn(tx);
+    });
+
+    const result = await processRewardClaim("sub-1", "user-1", 5);
+
+    expect(result).toBe(true);
+    expect(mockDb.transaction).toHaveBeenCalledTimes(1);
+    expect(invokeContract).toHaveBeenCalledWith(
+      "test-reward-contract",
+      "claim_reward",
+      expect.any(Array)
+    );
+  });
+
+  it("should throw when on-chain transaction fails", async () => {
+    const submissionChain = makeThenable([
+      {
+        id: "sub-1",
+        userId: "user-1",
+        score: 5,
+        rewardClaimed: false,
+        quizId: "quiz-1",
+      },
+    ]);
+    const quizChain = makeThenable([{ id: "quiz-1", courseId: "course-1" }]);
+    const userChain = makeThenable([
+      {
+        id: "user-1",
+        stellarAddress:
+          "GALICE0000000000000000000000000000000000000000000000000000000",
+      },
+    ]);
+
+    mockDb.select
+      .mockReturnValueOnce(submissionChain)
+      .mockReturnValueOnce(quizChain)
+      .mockReturnValueOnce(userChain);
+
+    vi.mocked(invokeContract).mockRejectedValue(new Error("Stellar error"));
+
+    await expect(
+      processRewardClaim("sub-1", "user-1", 5)
+    ).rejects.toThrow("Stellar error");
+  });
+});