diff --git a/src/routes/relayer.routes.js b/src/routes/relayer.routes.js
index 94bafc5..5411569 100644
--- a/src/routes/relayer.routes.js
+++ b/src/routes/relayer.routes.js
@@ -1,10 +1,12 @@
 const express = require('express');
 const router = express.Router();
 const { validate } = require('../middleware/validate.middleware');
-const { submitEscrowSchema } = require('../validation/schemas/escrow.schema');
+const { submitEscrowSchema, syncEscrowSchema } = require('../validation/schemas/escrow.schema');
 const { fundWalletSchema } = require('../validation/schemas/funding.schema');
 const { createWalletProvider } = require('../providers/wallet.provider');
 const { createFundingService } = require('../services/funding.service');
+const { createEscrowSyncService } = require('../services/escrow-sync.service');
+const { EscrowIntentRepository } = require('../repositories/escrow-intent.repository');
 
 // TODO: Import escrow service and horizon service (to be implemented in Phase 4)
 // const escrowService = require('../services/escrow.service');
@@ -14,6 +16,10 @@ const { createFundingService } = require('../services/funding.service');
 const walletProvider = createWalletProvider();
 const fundingService = createFundingService({ walletProvider });
 
+// Compose the on-chain synchronization service against the escrow intent repository.
+const escrowIntentRepository = new EscrowIntentRepository();
+const escrowSyncService = createEscrowSyncService({ escrowIntentRepository });
+
 /**
  * POST /submit-escrow
  * Endpoint for the WhatsApp bot to request a new escrow action.
@@ -42,6 +48,32 @@ router.post('/fund', validate(fundWalletSchema), async (req, res, next) => {
   }
 });
 
+/**
+ * POST /escrow/:id/sync
+ * Internal/webhook endpoint that bridges an EscrowIntent's off-chain record
+ * with the outcome of its on-chain transaction: on a reported SUCCESS it
+ * deterministically stamps the intent with the on-chain escrow id and
+ * transitions its status from PENDING to LOCKED. Not tied to a specific
+ * user session, so it is not gated by the `authenticate` middleware.
+ */
+router.post('/escrow/:id/sync', validate(syncEscrowSchema), async (req, res, next) => {
+  try {
+    const result = await escrowSyncService.syncEscrowOnChain({
+      escrowIntentId: req.params.id,
+      sorobanEscrowId: req.body.sorobanEscrowId,
+      status: req.body.status,
+    });
+
+    res.status(200).json({
+      success: true,
+      message: 'Escrow intent synchronization processed',
+      data: result,
+    });
+  } catch (error) {
+    next(error);
+  }
+});
+
 /**
  * GET /status/:txId
  * Endpoint to check the on-chain status of a previously submitted transaction.
diff --git a/src/services/escrow-sync.service.js b/src/services/escrow-sync.service.js
new file mode 100644
index 0000000..62c45fc
--- /dev/null
+++ b/src/services/escrow-sync.service.js
@@ -0,0 +1,59 @@
+const AppError = require('../errors/AppError');
+
+/**
+ * Factory function for the On-Chain Synchronization Service.
+ *
+ * Bridges the off-chain EscrowIntent record with the on-chain truth: given
+ * the outcome of a previously submitted transaction (reported by a webhook
+ * or internal callback, never polled directly here), it deterministically
+ * stamps the intent with the resulting Soroban escrow id and transitions its
+ * status from PENDING to LOCKED.
+ *
+ * @param {Object} deps - Dependencies
+ * @param {Object} deps.escrowIntentRepository - Escrow Intent Data Access Repository.
+ */
+const createEscrowSyncService = ({ escrowIntentRepository }) => {
+  /**
+   * Synchronizes an EscrowIntent with the outcome of its on-chain transaction.
+   * @param {Object} params - Synchronization parameters
+   * @param {string} params.escrowIntentId - The EscrowIntent to synchronize.
+   * @param {string} params.sorobanEscrowId - The on-chain escrow id to stamp.
+   * @param {string} params.status - The reported transaction status (e.g. 'SUCCESS').
+   * @returns {Promise<Object>} `{ escrowIntent, synchronized }`
+   */
+  const syncEscrowOnChain = async ({ escrowIntentId, sorobanEscrowId, status }) => {
+    const escrowIntent = await escrowIntentRepository.findById(escrowIntentId);
+    if (!escrowIntent) {
+      throw new AppError('Escrow intent not found', 404);
+    }
+
+    if (status !== 'SUCCESS') {
+      // Not a caller error: the reported transaction simply didn't succeed,
+      // so there is nothing to synchronize.
+      return { escrowIntent, synchronized: false };
+    }
+
+    if (escrowIntent.status === 'LOCKED') {
+      if (escrowIntent.sorobanEscrowId === sorobanEscrowId) {
+        // Duplicate delivery of the same synchronization event: idempotent no-op.
+        return { escrowIntent, synchronized: true };
+      }
+      throw new AppError('Escrow intent already locked with a different on-chain escrow id', 409);
+    }
+
+    if (escrowIntent.status !== 'PENDING') {
+      throw new AppError(`Escrow intent is not awaiting synchronization in its current state: ${escrowIntent.status}`, 409);
+    }
+
+    const updatedEscrowIntent = await escrowIntentRepository.update(escrowIntentId, {
+      sorobanEscrowId,
+      status: 'LOCKED',
+    });
+
+    return { escrowIntent: updatedEscrowIntent, synchronized: true };
+  };
+
+  return { syncEscrowOnChain };
+};
+
+module.exports = { createEscrowSyncService };
diff --git a/src/validation/schemas/escrow.schema.js b/src/validation/schemas/escrow.schema.js
index 75120f3..5383b59 100644
--- a/src/validation/schemas/escrow.schema.js
+++ b/src/validation/schemas/escrow.schema.js
@@ -22,4 +22,14 @@ const escrowActionSchema = z.object({
   }),
 });
 
-module.exports = { submitEscrowSchema, createEscrowSchema, escrowActionSchema };
+const syncEscrowSchema = z.object({
+  params: z.object({
+    id: z.string().min(1, "Escrow ID is required"),
+  }),
+  body: z.object({
+    sorobanEscrowId: z.string().min(1, "On-chain escrow id is required"),
+    status: z.enum(['SUCCESS', 'FAILED', 'NOT_FOUND', 'PENDING']),
+  }),
+});
+
+module.exports = { submitEscrowSchema, createEscrowSchema, escrowActionSchema, syncEscrowSchema };
diff --git a/tests/routes/relayer.routes.test.js b/tests/routes/relayer.routes.test.js
index 672855b..0f87414 100644
--- a/tests/routes/relayer.routes.test.js
+++ b/tests/routes/relayer.routes.test.js
@@ -1,5 +1,20 @@
 const express = require('express');
 const request = require('supertest');
+const AppError = require('../../src/errors/AppError');
+
+// Mock prisma so repository construction never touches a real database.
+jest.mock('../../src/clients/prisma.client', () => ({}));
+
+// Mock the escrow sync service so route tests never touch the database.
+jest.mock('../../src/services/escrow-sync.service', () => ({
+  createEscrowSyncService: jest.fn().mockReturnValue({
+    syncEscrowOnChain: jest.fn(),
+  }),
+}));
+
+const { createEscrowSyncService } = require('../../src/services/escrow-sync.service');
+const escrowSyncServiceMock = createEscrowSyncService();
+
 const relayerRoutes = require('../../src/routes/relayer.routes');
 const errorHandler = require('../../src/middleware/error.middleware');
 
@@ -13,6 +28,10 @@ describe('Relayer Routes', () => {
     app.use(errorHandler);
   });
 
+  beforeEach(() => {
+    jest.clearAllMocks();
+  });
+
   describe('POST /api/relayer/submit-escrow', () => {
     it('should fail validation when payload is missing', async () => {
       const res = await request(app).post('/api/relayer/submit-escrow').send({});
@@ -77,6 +96,51 @@ describe('Relayer Routes', () => {
     });
   });
 
+  describe('POST /api/relayer/escrow/:id/sync', () => {
+    it('should reject an invalid payload at the validation layer', async () => {
+      const res = await request(app)
+        .post('/api/relayer/escrow/intent-1/sync')
+        .send({ status: 'SUCCESS' }); // missing sorobanEscrowId
+
+      expect(res.status).toBe(400);
+      expect(res.body.error).toBe('VALIDATION_ERROR');
+      expect(escrowSyncServiceMock.syncEscrowOnChain).not.toHaveBeenCalled();
+    });
+
+    it('should synchronize a valid payload and return 200', async () => {
+      const serviceResult = {
+        escrowIntent: { id: 'intent-1', status: 'LOCKED', sorobanEscrowId: '42' },
+        synchronized: true,
+      };
+      escrowSyncServiceMock.syncEscrowOnChain.mockResolvedValue(serviceResult);
+
+      const res = await request(app)
+        .post('/api/relayer/escrow/intent-1/sync')
+        .send({ sorobanEscrowId: '42', status: 'SUCCESS' });
+
+      expect(res.status).toBe(200);
+      expect(res.body.success).toBe(true);
+      expect(res.body.data).toEqual(serviceResult);
+      expect(escrowSyncServiceMock.syncEscrowOnChain).toHaveBeenCalledWith({
+        escrowIntentId: 'intent-1',
+        sorobanEscrowId: '42',
+        status: 'SUCCESS',
+      });
+    });
+
+    it('should propagate service errors through the error handler', async () => {
+      escrowSyncServiceMock.syncEscrowOnChain.mockRejectedValue(new AppError('Escrow intent not found', 404));
+
+      const res = await request(app)
+        .post('/api/relayer/escrow/intent-1/sync')
+        .send({ sorobanEscrowId: '42', status: 'SUCCESS' });
+
+      expect(res.status).toBe(404);
+      expect(res.body.success).toBe(false);
+      expect(res.body.message).toBe('Escrow intent not found');
+    });
+  });
+
   describe('GET /api/relayer/status/:txId', () => {
     it('should hit scaffolded route and return txId', async () => {
       const res = await request(app).get('/api/relayer/status/12345');
diff --git a/tests/services/escrow-sync.service.test.js b/tests/services/escrow-sync.service.test.js
new file mode 100644
index 0000000..e1388d0
--- /dev/null
+++ b/tests/services/escrow-sync.service.test.js
@@ -0,0 +1,101 @@
+const { createEscrowSyncService } = require('../../src/services/escrow-sync.service');
+
+describe('Escrow Sync Service', () => {
+  let escrowSyncService;
+  let escrowIntentRepositoryMock;
+
+  const pendingIntent = {
+    id: 'intent-1',
+    buyerId: 'buyer-1',
+    sellerId: 'seller-1',
+    amount: 100,
+    status: 'PENDING',
+    sorobanEscrowId: null,
+  };
+
+  beforeEach(() => {
+    escrowIntentRepositoryMock = {
+      findById: jest.fn().mockResolvedValue({ ...pendingIntent }),
+      update: jest.fn().mockResolvedValue({ ...pendingIntent, status: 'LOCKED', sorobanEscrowId: '42' }),
+    };
+
+    escrowSyncService = createEscrowSyncService({ escrowIntentRepository: escrowIntentRepositoryMock });
+  });
+
+  it('should transition a PENDING intent to LOCKED and stamp the on-chain escrow id', async () => {
+    const result = await escrowSyncService.syncEscrowOnChain({
+      escrowIntentId: 'intent-1',
+      sorobanEscrowId: '42',
+      status: 'SUCCESS',
+    });
+
+    expect(escrowIntentRepositoryMock.update).toHaveBeenCalledWith('intent-1', {
+      sorobanEscrowId: '42',
+      status: 'LOCKED',
+    });
+    expect(result.synchronized).toBe(true);
+    expect(result.escrowIntent.status).toBe('LOCKED');
+    expect(result.escrowIntent.sorobanEscrowId).toBe('42');
+  });
+
+  it('should throw 404 when the escrow intent does not exist', async () => {
+    escrowIntentRepositoryMock.findById.mockResolvedValue(null);
+
+    await expect(
+      escrowSyncService.syncEscrowOnChain({ escrowIntentId: 'missing', sorobanEscrowId: '42', status: 'SUCCESS' })
+    ).rejects.toMatchObject({ statusCode: 404 });
+  });
+
+  it('should not mutate the record and report synchronized:false for a non-SUCCESS status', async () => {
+    const result = await escrowSyncService.syncEscrowOnChain({
+      escrowIntentId: 'intent-1',
+      sorobanEscrowId: '42',
+      status: 'FAILED',
+    });
+
+    expect(result.synchronized).toBe(false);
+    expect(result.escrowIntent.status).toBe('PENDING');
+    expect(escrowIntentRepositoryMock.update).not.toHaveBeenCalled();
+  });
+
+  it('should be idempotent when the same synchronization event is delivered twice', async () => {
+    escrowIntentRepositoryMock.findById.mockResolvedValue({
+      ...pendingIntent,
+      status: 'LOCKED',
+      sorobanEscrowId: '42',
+    });
+
+    const result = await escrowSyncService.syncEscrowOnChain({
+      escrowIntentId: 'intent-1',
+      sorobanEscrowId: '42',
+      status: 'SUCCESS',
+    });
+
+    expect(result.synchronized).toBe(true);
+    expect(escrowIntentRepositoryMock.update).not.toHaveBeenCalled();
+  });
+
+  it('should throw 409 when already LOCKED with a different on-chain escrow id', async () => {
+    escrowIntentRepositoryMock.findById.mockResolvedValue({
+      ...pendingIntent,
+      status: 'LOCKED',
+      sorobanEscrowId: '42',
+    });
+
+    await expect(
+      escrowSyncService.syncEscrowOnChain({ escrowIntentId: 'intent-1', sorobanEscrowId: '99', status: 'SUCCESS' })
+    ).rejects.toMatchObject({ statusCode: 409 });
+
+    expect(escrowIntentRepositoryMock.update).not.toHaveBeenCalled();
+  });
+
+  it('should throw 409 when the intent is in an unexpected lifecycle state', async () => {
+    escrowIntentRepositoryMock.findById.mockResolvedValue({ ...pendingIntent, status: 'CANCELLED' });
+
+    await expect(
+      escrowSyncService.syncEscrowOnChain({ escrowIntentId: 'intent-1', sorobanEscrowId: '42', status: 'SUCCESS' })
+    ).rejects.toMatchObject({ statusCode: 409 });
+
+    expect(escrowIntentRepositoryMock.update).not.toHaveBeenCalled();
+  });
+});
diff --git a/tests/validation/schemas/escrow.schema.test.js b/tests/validation/schemas/escrow.schema.test.js
index 84ca696..81f31da 100644
--- a/tests/validation/schemas/escrow.schema.test.js
+++ b/tests/validation/schemas/escrow.schema.test.js
@@ -1,4 +1,4 @@
-const { createEscrowSchema, submitEscrowSchema, escrowActionSchema } = require('../../../src/validation/schemas/escrow.schema');
+const { createEscrowSchema, submitEscrowSchema, escrowActionSchema, syncEscrowSchema } = require('../../../src/validation/schemas/escrow.schema');
 
 describe('Escrow Schemas', () => {
   describe('createEscrowSchema', () => {
@@ -59,4 +59,34 @@ describe('Escrow Schemas', () => {
       expect(result.success).toBe(false);
     });
   });
+
+  describe('syncEscrowSchema', () => {
+    it('should validate a correct synchronization payload', () => {
+      const payload = { params: { id: 'intent-1' }, body: { sorobanEscrowId: '42', status: 'SUCCESS' } };
+      const result = syncEscrowSchema.safeParse(payload);
+      expect(result.success).toBe(true);
+    });
+
+    it('should accept any recognized transaction status', () => {
+      ['SUCCESS', 'FAILED', 'NOT_FOUND', 'PENDING'].forEach((status) => {
+        const result = syncEscrowSchema.safeParse({ params: { id: 'intent-1' }, body: { sorobanEscrowId: '42', status } });
+        expect(result.success).toBe(true);
+      });
+    });
+
+    it('should fail when sorobanEscrowId is missing', () => {
+      const result = syncEscrowSchema.safeParse({ params: { id: 'intent-1' }, body: { status: 'SUCCESS' } });
+      expect(result.success).toBe(false);
+    });
+
+    it('should fail for an unrecognized status', () => {
+      const result = syncEscrowSchema.safeParse({ params: { id: 'intent-1' }, body: { sorobanEscrowId: '42', status: 'BOGUS' } });
+      expect(result.success).toBe(false);
+    });
+
+    it('should fail when escrow id param is missing', () => {
+      const result = syncEscrowSchema.safeParse({ params: {}, body: { sorobanEscrowId: '42', status: 'SUCCESS' } });
+      expect(result.success).toBe(false);
+    });
+  });
 });