diff --git a/src/lib/services/yachts.service.ts b/src/lib/services/yachts.service.ts
index 864083e..7652dcd 100644
--- a/src/lib/services/yachts.service.ts
+++ b/src/lib/services/yachts.service.ts
@@ -1,4 +1,4 @@
-import { and, eq } from 'drizzle-orm';
+import { and, eq, sql } from 'drizzle-orm';
 import { db } from '@/lib/db';
 import { yachts, yachtOwnershipHistory, clients } from '@/lib/db/schema';
 import { companies } from '@/lib/db/schema/companies';
@@ -8,7 +8,11 @@ import { emitToRoom } from '@/lib/socket/server';
 import { diffEntity } from '@/lib/entity-diff';
 import { withTransaction } from '@/lib/db/utils';
 import type { z } from 'zod';
-import type { createYachtSchema, UpdateYachtInput } from '@/lib/validators/yachts';
+import type {
+  createYachtSchema,
+  UpdateYachtInput,
+  TransferOwnershipInput,
+} from '@/lib/validators/yachts';
 
 type CreateYachtInput = z.input<typeof createYachtSchema>;
 
@@ -184,3 +188,78 @@ export async function archiveYacht(id: string, portId: string, meta: AuditMeta)
 
   emitToRoom(`port:${portId}`, 'yacht:archived', { yachtId: id });
 }
+
+export async function transferOwnership(
+  yachtId: string,
+  portId: string,
+  data: TransferOwnershipInput,
+  meta: AuditMeta,
+) {
+  return await withTransaction(async (tx) => {
+    const yacht = await tx.query.yachts.findFirst({
+      where: and(eq(yachts.id, yachtId), eq(yachts.portId, portId)),
+    });
+    if (!yacht) throw new NotFoundError('Yacht');
+
+    if (
+      yacht.currentOwnerType === data.newOwner.type &&
+      yacht.currentOwnerId === data.newOwner.id
+    ) {
+      throw new ValidationError('same owner — nothing to transfer');
+    }
+
+    await assertOwnerExists(portId, data.newOwner, tx);
+
+    // Close the currently-active history row
+    await tx
+      .update(yachtOwnershipHistory)
+      .set({ endDate: data.effectiveDate })
+      .where(
+        and(
+          eq(yachtOwnershipHistory.yachtId, yachtId),
+          sql`${yachtOwnershipHistory.endDate} IS NULL`,
+        ),
+      );
+
+    // Open new row
+    await tx.insert(yachtOwnershipHistory).values({
+      yachtId,
+      ownerType: data.newOwner.type,
+      ownerId: data.newOwner.id,
+      startDate: data.effectiveDate,
+      endDate: null,
+      transferReason: data.transferReason ?? null,
+      transferNotes: data.transferNotes ?? null,
+      createdBy: meta.userId,
+    });
+
+    // Update denormalized current-owner columns
+    const [updated] = await tx
+      .update(yachts)
+      .set({
+        currentOwnerType: data.newOwner.type,
+        currentOwnerId: data.newOwner.id,
+        updatedAt: new Date(),
+      })
+      .where(eq(yachts.id, yachtId))
+      .returning();
+
+    void createAuditLog({
+      userId: meta.userId,
+      portId,
+      action: 'update',
+      entityType: 'yacht',
+      entityId: yachtId,
+      newValue: { ownerTransferTo: data.newOwner, reason: data.transferReason },
+      ipAddress: meta.ipAddress,
+      userAgent: meta.userAgent,
+    });
+
+    emitToRoom(`port:${portId}`, 'yacht:ownership_transferred', {
+      yachtId,
+      newOwner: data.newOwner,
+    });
+
+    return updated!;
+  });
+}
diff --git a/src/lib/socket/events.ts b/src/lib/socket/events.ts
index d43e748..6f6068f 100644
--- a/src/lib/socket/events.ts
+++ b/src/lib/socket/events.ts
@@ -81,6 +81,10 @@ export interface ServerToClientEvents {
   'yacht:created': (payload: { yachtId: string }) => void;
   'yacht:updated': (payload: { yachtId: string; changedFields: string[] }) => void;
   'yacht:archived': (payload: { yachtId: string }) => void;
+  'yacht:ownership_transferred': (payload: {
+    yachtId: string;
+    newOwner: { type: 'client' | 'company'; id: string };
+  }) => void;
 
   // Document events
   'document:created': (payload: { documentId: string; type?: string; interestId?: string }) => void;
diff --git a/tests/integration/ownership-transfer.test.ts b/tests/integration/ownership-transfer.test.ts
new file mode 100644
index 0000000..1684878
--- /dev/null
+++ b/tests/integration/ownership-transfer.test.ts
@@ -0,0 +1,74 @@
+import { describe, it, expect } from 'vitest';
+import { transferOwnership } from '@/lib/services/yachts.service';
+import { makeClient, makePort, makeYacht, makeAuditMeta } from '../helpers/factories';
+import { db } from '@/lib/db';
+import { yachtOwnershipHistory, yachts } from '@/lib/db/schema';
+import { eq } from 'drizzle-orm';
+
+describe('transferOwnership', () => {
+  it('closes prior history row and opens a new one atomically', async () => {
+    const port = await makePort();
+    const clientA = await makeClient({ portId: port.id });
+    const clientB = await makeClient({ portId: port.id });
+    const yacht = await makeYacht({ portId: port.id, ownerType: 'client', ownerId: clientA.id });
+
+    await transferOwnership(
+      yacht.id,
+      port.id,
+      {
+        newOwner: { type: 'client', id: clientB.id },
+        effectiveDate: new Date(),
+        transferReason: 'sale',
+      },
+      makeAuditMeta(),
+    );
+
+    const history = await db
+      .select()
+      .from(yachtOwnershipHistory)
+      .where(eq(yachtOwnershipHistory.yachtId, yacht.id));
+    expect(history).toHaveLength(2);
+    const [prior, current] = history.sort((a, b) => a.startDate.getTime() - b.startDate.getTime());
+    expect(prior!.endDate).not.toBeNull();
+    expect(current!.endDate).toBeNull();
+    expect(current!.ownerId).toBe(clientB.id);
+
+    const updatedYacht = await db.query.yachts.findFirst({ where: eq(yachts.id, yacht.id) });
+    expect(updatedYacht!.currentOwnerId).toBe(clientB.id);
+  });
+
+  it('rejects when newOwner = currentOwner (no-op)', async () => {
+    const port = await makePort();
+    const client = await makeClient({ portId: port.id });
+    const yacht = await makeYacht({ portId: port.id, ownerType: 'client', ownerId: client.id });
+
+    await expect(
+      transferOwnership(
+        yacht.id,
+        port.id,
+        { newOwner: { type: 'client', id: client.id }, effectiveDate: new Date() },
+        makeAuditMeta(),
+      ),
+    ).rejects.toThrow(/same owner/i);
+  });
+
+  it('partial unique index prevents concurrent double-open ownership rows', async () => {
+    // Prior to the atomic close-then-open, a naive impl would insert a new open row
+    // without closing the old one. Verify this would be blocked at the DB level.
+    const port = await makePort();
+    const clientA = await makeClient({ portId: port.id });
+    const clientB = await makeClient({ portId: port.id });
+    const yacht = await makeYacht({ portId: port.id, ownerType: 'client', ownerId: clientA.id });
+
+    await expect(
+      db.insert(yachtOwnershipHistory).values({
+        yachtId: yacht.id,
+        ownerType: 'client',
+        ownerId: clientB.id,
+        startDate: new Date(),
+        endDate: null,
+        createdBy: 'test',
+      }),
+    ).rejects.toThrow(/duplicate key/i);
+  });
+});