diff --git a/src/modules/ownership/ownership-update-log.test.ts b/src/modules/ownership/ownership-update-log.test.ts
new file mode 100644
index 0000000..cfc264c
--- /dev/null
+++ b/src/modules/ownership/ownership-update-log.test.ts
@@ -0,0 +1,112 @@
+import { updateOwnership } from './ownership.service';
+import { logger } from '../../utils/logger.utils';
+import { prisma } from '../../utils/prisma.utils';
+
+jest.mock('../../utils/prisma.utils', () => ({
+  prisma: {
+    keyOwnership: {
+      findFirst: jest.fn(),
+      upsert: jest.fn(),
+    },
+  },
+}));
+
+jest.mock('../../utils/logger.utils', () => ({
+  logger: {
+    debug: jest.fn(),
+    info: jest.fn(),
+    warn: jest.fn(),
+    error: jest.fn(),
+  },
+  HTTP_STATUS: {},
+}));
+
+const mockPrisma = prisma as jest.Mocked<typeof prisma>;
+
+function mockOwnership(balance: number) {
+  return {
+    id: 'own-1',
+    ownerAddress: 'GTEST1234GTEST1234GTEST1234GTEST1234GTEST1234GTEST12345',
+    creatorId: 'creator-1',
+    balance: BigInt(balance),
+    createdAt: new Date('2024-01-01'),
+    updatedAt: new Date('2024-01-02'),
+  };
+}
+
+describe('updateOwnership — structured debug log', () => {
+  const ownerAddress = 'GTEST1234GTEST1234GTEST1234GTEST1234GTEST1234GTEST12345';
+  const creatorId = 'creator-1';
+
+  beforeEach(() => {
+    jest.clearAllMocks();
+  });
+
+  it('emits debug log with all six required fields after successful update', async () => {
+    (mockPrisma.keyOwnership.findFirst as jest.Mock).mockResolvedValue(mockOwnership(10));
+    (mockPrisma.keyOwnership.upsert as jest.Mock).mockResolvedValue(mockOwnership(15));
+
+    await updateOwnership(ownerAddress, creatorId, 5, {
+      event_type: 'buy',
+      ledger_sequence: 42,
+    });
+
+    expect(logger.debug).toHaveBeenCalledTimes(1);
+    const [logFields, _msg] = (logger.debug as jest.Mock).mock.calls[0];
+
+    expect(logFields).toMatchObject({
+      creator_id: creatorId,
+      previous_balance: 10,
+      new_balance: 15,
+      event_type: 'buy',
+      ledger_sequence: 42,
+    });
+    expect(typeof logFields.wallet_address).toBe('string');
+    expect(logFields.wallet_address).not.toBe(ownerAddress); // masked
+    expect(logFields.wallet_address).toContain('GTES');  // first 4
+    expect(logFields.wallet_address).toContain('2345');  // last 4
+  });
+
+  it('masks wallet address to first 4 and last 4 characters', async () => {
+    (mockPrisma.keyOwnership.findFirst as jest.Mock).mockResolvedValue(null);
+    (mockPrisma.keyOwnership.upsert as jest.Mock).mockResolvedValue(mockOwnership(5));
+
+    await updateOwnership(ownerAddress, creatorId, 5);
+
+    const [logFields] = (logger.debug as jest.Mock).mock.calls[0];
+    // masked address should not be the full address
+    expect(logFields.wallet_address.length).toBeLessThan(ownerAddress.length);
+    expect(logFields.wallet_address.startsWith(ownerAddress.slice(0, 4))).toBe(true);
+    expect(logFields.wallet_address.endsWith(ownerAddress.slice(-4))).toBe(true);
+  });
+
+  it('sets previous_balance to 0 when record is new (create path)', async () => {
+    (mockPrisma.keyOwnership.findFirst as jest.Mock).mockResolvedValue(null);
+    (mockPrisma.keyOwnership.upsert as jest.Mock).mockResolvedValue(mockOwnership(10));
+
+    await updateOwnership(ownerAddress, creatorId, 10, { event_type: 'buy' });
+
+    const [logFields] = (logger.debug as jest.Mock).mock.calls[0];
+    expect(logFields.previous_balance).toBe(0);
+    expect(logFields.new_balance).toBe(10);
+  });
+
+  it('correctly logs sell event type', async () => {
+    (mockPrisma.keyOwnership.findFirst as jest.Mock).mockResolvedValue(mockOwnership(20));
+    (mockPrisma.keyOwnership.upsert as jest.Mock).mockResolvedValue(mockOwnership(15));
+
+    await updateOwnership(ownerAddress, creatorId, -5, { event_type: 'sell', ledger_sequence: 100 });
+
+    const [logFields] = (logger.debug as jest.Mock).mock.calls[0];
+    expect(logFields.event_type).toBe('sell');
+    expect(logFields.ledger_sequence).toBe(100);
+  });
+
+  it('does not emit log when upsert throws', async () => {
+    (mockPrisma.keyOwnership.findFirst as jest.Mock).mockResolvedValue(null);
+    (mockPrisma.keyOwnership.upsert as jest.Mock).mockRejectedValue(new Error('DB error'));
+
+    await expect(updateOwnership(ownerAddress, creatorId, 5)).rejects.toThrow('DB error');
+    expect(logger.debug).not.toHaveBeenCalled();
+  });
+});
diff --git a/src/modules/ownership/ownership.service.ts b/src/modules/ownership/ownership.service.ts
index ca8a9ec..86863d5 100644
--- a/src/modules/ownership/ownership.service.ts
+++ b/src/modules/ownership/ownership.service.ts
@@ -1,8 +1,14 @@
 import { prisma } from '../../utils/prisma.utils';
+import { logger } from '../../utils/logger.utils';
 import { OwnershipQueryType } from './ownership.schemas';
 
 type KeyOwnership = NonNullable<Awaited<ReturnType<typeof prisma.keyOwnership.findFirst>>>;
 
+function maskAddress(address: string): string {
+  if (address.length <= 8) return address;
+  return `${address.slice(0, 4)}…${address.slice(-4)}`;
+}
+
 export async function fetchOwnership(
     query: OwnershipQueryType
 ): Promise<KeyOwnership[]> {
@@ -18,12 +24,24 @@ export async function fetchOwnership(
     });
 }
 
+export interface OwnershipUpdateContext {
+  event_type?: 'buy' | 'sell';
+  ledger_sequence?: number;
+}
+
 export async function updateOwnership(
     ownerAddress: string,
     creatorId: string,
-    balanceChange: number
+    balanceChange: number,
+    ctx: OwnershipUpdateContext = {},
 ): Promise<KeyOwnership> {
-    return prisma.keyOwnership.upsert({
+    const existing = await prisma.keyOwnership.findFirst({
+        where: { ownerAddress, creatorId },
+        select: { balance: true },
+    });
+    const previousBalance = existing ? Number(existing.balance) : 0;
+
+    const result = await prisma.keyOwnership.upsert({
         where: {
             ownerAddress_creatorId: {
                 ownerAddress,
@@ -39,4 +57,15 @@ export async function updateOwnership(
             balance: balanceChange,
         },
     });
+
+    logger.debug({
+        creator_id: creatorId,
+        wallet_address: maskAddress(ownerAddress),
+        previous_balance: previousBalance,
+        new_balance: Number(result.balance),
+        event_type: ctx.event_type,
+        ledger_sequence: ctx.ledger_sequence,
+    }, 'Ownership read model updated');
+
+    return result;
 }