diff --git a/apps/backend/src/services/auth.service.ts b/apps/backend/src/services/auth.service.ts
index d4970c6..a2f8a4c 100644
--- a/apps/backend/src/services/auth.service.ts
+++ b/apps/backend/src/services/auth.service.ts
@@ -1,4 +1,5 @@
 import { supabaseAdmin } from "../config/supabase.js";
+import { prisma } from "../config/database.js";
 import { AppError } from "../utils/AppError.js";
 import { logger } from "../utils/logger.js";
 import type { SignupInput, LoginInput, RefreshInput } from "../schemas/auth.schema.js";
@@ -42,6 +43,32 @@ export class AuthService {
       throw new AppError(400, "SIGNUP_FAILED", "Failed to create account");
     }
     const userId = data.user.id;
+    const email = data.user.email!;
+
+    try {
+      await prisma.user.create({
+        data: {
+          id: userId,
+          email,
+          displayName: input.displayName,
+        },
+      });
+    } catch (error) {
+      logger.error({ error, userId }, "Failed to create application user after signup");
+      try {
+        await supabaseAdmin.auth.admin.deleteUser(userId);
+      } catch (cleanupError) {
+        logger.warn(
+          { cleanupError, userId },
+          "Failed to roll back Supabase user after signup profile creation failed",
+        );
+      }
+      throw new AppError(
+        500,
+        "SIGNUP_PROFILE_FAILED",
+        "Failed to create account profile",
+      );
+    }
 
     // Warm up a per-user model in the background; auth should not fail if ML training is down.
     void mlService.trainUserModel(userId).catch((error) => {
@@ -52,7 +79,7 @@ export class AuthService {
     return {
       user: {
         id: userId,
-        email: data.user.email!,
+        email,
       },
       tokens: {
         accessToken: data.session.access_token,
diff --git a/apps/backend/tests/auth.test.ts b/apps/backend/tests/auth.test.ts
new file mode 100644
index 0000000..8d12819
--- /dev/null
+++ b/apps/backend/tests/auth.test.ts
@@ -0,0 +1,116 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+vi.mock("../src/config/database.js", () => ({
+  isDatabaseHealthy: vi.fn().mockResolvedValue(true),
+  prisma: {
+    user: {
+      create: vi.fn(),
+    },
+  },
+}));
+
+vi.mock("../src/config/supabase.js", () => ({
+  supabaseAdmin: {
+    auth: {
+      signUp: vi.fn(),
+      admin: {
+        deleteUser: vi.fn(),
+      },
+    },
+  },
+}));
+
+vi.mock("../src/services/ml.service.js", () => ({
+  mlService: {
+    trainUserModel: vi.fn().mockResolvedValue({
+      user_id: "550e8400-e29b-41d4-a716-446655440000",
+      interaction_count: 0,
+      is_cold_start: true,
+      message: "queued",
+    }),
+  },
+}));
+
+vi.mock("../src/config/sentry.js", () => ({
+  captureMlFailure: vi.fn(),
+}));
+
+vi.mock("../src/utils/logger.js", () => ({
+  logger: { debug: vi.fn(), info: vi.fn(), error: vi.fn(), warn: vi.fn() },
+}));
+
+import { prisma } from "../src/config/database.js";
+import { supabaseAdmin } from "../src/config/supabase.js";
+import { mlService } from "../src/services/ml.service.js";
+import { authService } from "../src/services/auth.service.js";
+
+const USER_ID = "550e8400-e29b-41d4-a716-446655440000";
+const EMAIL = "new-user@snacktrack.dev";
+const SIGNUP_INPUT = {
+  email: EMAIL,
+  password: "Password123",
+  displayName: "New User",
+};
+
+function mockSuccessfulSupabaseSignup() {
+  vi.mocked(supabaseAdmin.auth.signUp).mockResolvedValue({
+    data: {
+      user: {
+        id: USER_ID,
+        email: EMAIL,
+      },
+      session: {
+        access_token: "access-token",
+        refresh_token: "refresh-token",
+        expires_in: 3600,
+        expires_at: 1234567890,
+      },
+    },
+    error: null,
+  } as never);
+}
+
+describe("AuthService", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    mockSuccessfulSupabaseSignup();
+    vi.mocked(prisma.user.create).mockResolvedValue({
+      id: USER_ID,
+      email: EMAIL,
+      displayName: SIGNUP_INPUT.displayName,
+    } as never);
+    vi.mocked(supabaseAdmin.auth.admin.deleteUser).mockResolvedValue({
+      data: null,
+      error: null,
+    } as never);
+  });
+
+  describe("signUp", () => {
+    it("creates the application user row before returning signup success", async () => {
+      const result = await authService.signUp(SIGNUP_INPUT);
+
+      expect(prisma.user.create).toHaveBeenCalledWith({
+        data: {
+          id: USER_ID,
+          email: EMAIL,
+          displayName: SIGNUP_INPUT.displayName,
+        },
+      });
+      expect(result.user).toEqual({ id: USER_ID, email: EMAIL });
+      expect(result.tokens.accessToken).toBe("access-token");
+      expect(mlService.trainUserModel).toHaveBeenCalledWith(USER_ID);
+    });
+
+    it("rolls back the Supabase auth user when application user creation fails", async () => {
+      vi.mocked(prisma.user.create).mockRejectedValue(new Error("database unavailable"));
+
+      await expect(authService.signUp(SIGNUP_INPUT)).rejects.toMatchObject({
+        statusCode: 500,
+        code: "SIGNUP_PROFILE_FAILED",
+      });
+
+      expect(supabaseAdmin.auth.admin.deleteUser).toHaveBeenCalledWith(USER_ID);
+      expect(mlService.trainUserModel).not.toHaveBeenCalled();
+    });
+  });
+});