diff --git a/prisma/migrations/20260429000000_add_transaction_audit_log/migration.sql b/prisma/migrations/20260429000000_add_transaction_audit_log/migration.sql
new file mode 100644
index 0000000..db614fd
--- /dev/null
+++ b/prisma/migrations/20260429000000_add_transaction_audit_log/migration.sql
@@ -0,0 +1,29 @@
+-- CreateTable
+CREATE TABLE "transaction_audit_logs" (
+    "id" TEXT NOT NULL,
+    "transaction_id" TEXT NOT NULL,
+    "actor_id" TEXT,
+    "action" TEXT NOT NULL,
+    "previous_data" JSONB,
+    "new_data" JSONB,
+    "ip_address" TEXT,
+    "user_agent" TEXT,
+    "created_at" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "transaction_audit_logs_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE INDEX "transaction_audit_logs_transaction_id_created_at_idx" ON "transaction_audit_logs"("transaction_id", "created_at");
+
+-- CreateIndex
+CREATE INDEX "transaction_audit_logs_actor_id_idx" ON "transaction_audit_logs"("actor_id");
+
+-- CreateIndex
+CREATE INDEX "transaction_audit_logs_action_idx" ON "transaction_audit_logs"("action");
+
+-- AddForeignKey
+ALTER TABLE "transaction_audit_logs" ADD CONSTRAINT "transaction_audit_logs_transaction_id_fkey" FOREIGN KEY ("transaction_id") REFERENCES "transactions"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "transaction_audit_logs" ADD CONSTRAINT "transaction_audit_logs_actor_id_fkey" FOREIGN KEY ("actor_id") REFERENCES "users"("id") ON DELETE SET NULL ON UPDATE CASCADE;
diff --git a/src/transactions/transaction-audit.service.spec.ts b/src/transactions/transaction-audit.service.spec.ts
new file mode 100644
index 0000000..6de09bd
--- /dev/null
+++ b/src/transactions/transaction-audit.service.spec.ts
@@ -0,0 +1,59 @@
+import { TransactionAuditService } from './transaction-audit.service';
+
+const mockPrisma = {
+  transactionAuditLog: {
+    create: jest.fn(),
+    findMany: jest.fn(),
+  },
+};
+
+describe('TransactionAuditService', () => {
+  let service: TransactionAuditService;
+
+  beforeEach(() => {
+    jest.clearAllMocks();
+    service = new TransactionAuditService(mockPrisma as any);
+  });
+
+  it('creates an audit log entry with correct fields', async () => {
+    mockPrisma.transactionAuditLog.create.mockResolvedValue({ id: 'log-1' });
+
+    await service.log('tx-1', 'CREATED', null, { amount: 100 }, { actorId: 'user-1', ipAddress: '127.0.0.1' });
+
+    expect(mockPrisma.transactionAuditLog.create).toHaveBeenCalledWith({
+      data: expect.objectContaining({
+        transactionId: 'tx-1',
+        action: 'CREATED',
+        previousData: undefined,
+        newData: { amount: 100 },
+        actorId: 'user-1',
+        ipAddress: '127.0.0.1',
+      }),
+    });
+  });
+
+  it('creates a log with null actor for system actions', async () => {
+    mockPrisma.transactionAuditLog.create.mockResolvedValue({ id: 'log-2' });
+
+    await service.log('tx-1', 'UPDATED', { status: 'PENDING' }, { status: 'COMPLETED' });
+
+    expect(mockPrisma.transactionAuditLog.create).toHaveBeenCalledWith({
+      data: expect.objectContaining({ actorId: undefined }),
+    });
+  });
+
+  it('returns audit logs ordered by createdAt asc', async () => {
+    const logs = [{ id: 'log-1', action: 'CREATED' }];
+    mockPrisma.transactionAuditLog.findMany.mockResolvedValue(logs);
+
+    const result = await service.findByTransaction('tx-1');
+
+    expect(mockPrisma.transactionAuditLog.findMany).toHaveBeenCalledWith(
+      expect.objectContaining({
+        where: { transactionId: 'tx-1' },
+        orderBy: { createdAt: 'asc' },
+      }),
+    );
+    expect(result).toBe(logs);
+  });
+});
diff --git a/src/transactions/transaction-audit.service.ts b/src/transactions/transaction-audit.service.ts
new file mode 100644
index 0000000..c0097ee
--- /dev/null
+++ b/src/transactions/transaction-audit.service.ts
@@ -0,0 +1,51 @@
+import { Injectable } from '@nestjs/common';
+import { PrismaService } from '../database/prisma.service';
+
+export interface AuditContext {
+  actorId?: string;
+  ipAddress?: string;
+  userAgent?: string;
+}
+
+@Injectable()
+export class TransactionAuditService {
+  constructor(private readonly prisma: PrismaService) {}
+
+  async log(
+    transactionId: string,
+    action: string,
+    previousData: object | null,
+    newData: object | null,
+    ctx: AuditContext = {},
+  ) {
+    return this.prisma.transactionAuditLog.create({
+      data: {
+        transactionId,
+        action,
+        previousData: previousData ?? undefined,
+        newData: newData ?? undefined,
+        actorId: ctx.actorId,
+        ipAddress: ctx.ipAddress,
+        userAgent: ctx.userAgent,
+      },
+    });
+  }
+
+  async findByTransaction(transactionId: string) {
+    return this.prisma.transactionAuditLog.findMany({
+      where: { transactionId },
+      orderBy: { createdAt: 'asc' },
+      select: {
+        id: true,
+        action: true,
+        previousData: true,
+        newData: true,
+        ipAddress: true,
+        createdAt: true,
+        actor: {
+          select: { id: true, firstName: true, lastName: true, email: true },
+        },
+      },
+    });
+  }
+}