diff --git a/apps/api/src/admin/admin.service.spec.ts b/apps/api/src/admin/admin.service.spec.ts
index 2a9c6fd..647b5c9 100644
--- a/apps/api/src/admin/admin.service.spec.ts
+++ b/apps/api/src/admin/admin.service.spec.ts
@@ -24,7 +24,15 @@ describe("AdminService", () => {
     workspaceMember: {
       create: vi.fn(),
     },
-    $transaction: vi.fn(),
+    session: {
+      deleteMany: vi.fn(),
+    },
+    $transaction: vi.fn(async (ops) => {
+      if (typeof ops === "function") {
+        return ops(mockPrismaService);
+      }
+      return Promise.all(ops);
+    }),
   };
 
   const mockAdminId = "550e8400-e29b-41d4-a716-446655440001";
@@ -82,10 +90,6 @@ describe("AdminService", () => {
     service = module.get<AdminService>(AdminService);
 
     vi.clearAllMocks();
-
-    mockPrismaService.$transaction.mockImplementation(async (fn: (tx: unknown) => unknown) => {
-      return fn(mockPrismaService);
-    });
   });
 
   it("should be defined", () => {
@@ -325,12 +329,13 @@ describe("AdminService", () => {
   });
 
   describe("deactivateUser", () => {
-    it("should set deactivatedAt on the user", async () => {
+    it("should set deactivatedAt and invalidate sessions", async () => {
       mockPrismaService.user.findUnique.mockResolvedValue(mockUser);
       mockPrismaService.user.update.mockResolvedValue({
         ...mockUser,
         deactivatedAt: new Date(),
       });
+      mockPrismaService.session.deleteMany.mockResolvedValue({ count: 3 });
 
       const result = await service.deactivateUser(mockUserId);
 
@@ -341,6 +346,7 @@ describe("AdminService", () => {
           data: { deactivatedAt: expect.any(Date) },
         })
       );
+      expect(mockPrismaService.session.deleteMany).toHaveBeenCalledWith({ where: { userId: mockUserId } });
     });
 
     it("should throw NotFoundException if user does not exist", async () => {
diff --git a/apps/api/src/admin/admin.service.ts b/apps/api/src/admin/admin.service.ts
index e228ad6..b4add35 100644
--- a/apps/api/src/admin/admin.service.ts
+++ b/apps/api/src/admin/admin.service.ts
@@ -192,19 +192,22 @@ export class AdminService {
       throw new BadRequestException(`User ${id} is already deactivated`);
     }
 
-    const user = await this.prisma.user.update({
-      where: { id },
-      data: { deactivatedAt: new Date() },
-      include: {
-        workspaceMemberships: {
-          include: {
-            workspace: { select: { id: true, name: true } },
+    const [user] = await this.prisma.$transaction([
+      this.prisma.user.update({
+        where: { id },
+        data: { deactivatedAt: new Date() },
+        include: {
+          workspaceMemberships: {
+            include: {
+              workspace: { select: { id: true, name: true } },
+            },
           },
         },
-      },
-    });
+      }),
+      this.prisma.session.deleteMany({ where: { userId: id } }),
+    ]);
 
-    this.logger.log(`User deactivated: ${id}`);
+    this.logger.log(`User deactivated and sessions invalidated: ${id}`);
 
     return {
       id: user.id,