diff --git a/backend/app/api/routes/users.py b/backend/app/api/routes/users.py
index 6429818458..84b32e6ff7 100644
--- a/backend/app/api/routes/users.py
+++ b/backend/app/api/routes/users.py
@@ -2,6 +2,7 @@
 from typing import Any
 
 from fastapi import APIRouter, Depends, HTTPException
+from fastapi.responses import JSONResponse
 from sqlmodel import col, delete, func, select
 
 from app import crud
@@ -13,6 +14,7 @@
 from app.core.config import settings
 from app.core.security import get_password_hash, verify_password
 from app.models import (
+    ConflictError,
     Item,
     Message,
     UpdatePassword,
@@ -49,7 +51,15 @@ def read_users(session: SessionDep, skip: int = 0, limit: int = 100) -> Any:
 
 
 @router.post(
-    "/", dependencies=[Depends(get_current_active_superuser)], response_model=UserPublic
+    "/",
+    dependencies=[Depends(get_current_active_superuser)],
+    response_model=UserPublic,
+    responses={
+        409: {
+            "model": ConflictError,
+            "description": "Duplicate email or username",
+        }
+    },
 )
 def create_user(*, session: SessionDep, user_in: UserCreate) -> Any:
     """
@@ -57,9 +67,9 @@ def create_user(*, session: SessionDep, user_in: UserCreate) -> Any:
     """
     user = crud.get_user_by_email(session=session, email=user_in.email)
     if user:
-        raise HTTPException(
-            status_code=400,
-            detail="The user with this email already exists in the system.",
+        return JSONResponse(
+            status_code=409,
+            content={"error": "conflict", "field": "email", "message": "Already in use"},
         )
 
     user = crud.create_user(session=session, user_create=user_in)
@@ -139,16 +149,25 @@ def delete_user_me(session: SessionDep, current_user: CurrentUser) -> Any:
     return Message(message="User deleted successfully")
 
 
-@router.post("/signup", response_model=UserPublic)
+@router.post(
+    "/signup",
+    response_model=UserPublic,
+    responses={
+        409: {
+            "model": ConflictError,
+            "description": "Duplicate email or username",
+        }
+    },
+)
 def register_user(session: SessionDep, user_in: UserRegister) -> Any:
     """
     Create new user without the need to be logged in.
     """
     user = crud.get_user_by_email(session=session, email=user_in.email)
     if user:
-        raise HTTPException(
-            status_code=400,
-            detail="The user with this email already exists in the system",
+        return JSONResponse(
+            status_code=409,
+            content={"error": "conflict", "field": "email", "message": "Already in use"},
         )
     user_create = UserCreate.model_validate(user_in)
     user = crud.create_user(session=session, user_create=user_create)
diff --git a/backend/app/models.py b/backend/app/models.py
index 2389b4a532..51d22f7c99 100644
--- a/backend/app/models.py
+++ b/backend/app/models.py
@@ -1,5 +1,7 @@
 import uuid
 
+from typing import Literal
+
 from pydantic import EmailStr
 from sqlmodel import Field, Relationship, SQLModel
 
@@ -111,3 +113,9 @@ class TokenPayload(SQLModel):
 class NewPassword(SQLModel):
     token: str
     new_password: str = Field(min_length=8, max_length=40)
+
+
+class ConflictError(SQLModel):
+    error: Literal["conflict"]
+    field: str
+    message: str
diff --git a/backend/tests/api/routes/test_users.py b/backend/tests/api/routes/test_users.py
index 39e053e554..03d74b4dc0 100644
--- a/backend/tests/api/routes/test_users.py
+++ b/backend/tests/api/routes/test_users.py
@@ -127,9 +127,12 @@ def test_create_user_existing_username(
         headers=superuser_token_headers,
         json=data,
     )
-    created_user = r.json()
-    assert r.status_code == 400
-    assert "_id" not in created_user
+    assert r.status_code == 409
+    assert r.json() == {
+        "error": "conflict",
+        "field": "email",
+        "message": "Already in use",
+    }
 
 
 def test_create_user_by_normal_user(
@@ -316,8 +319,12 @@ def test_register_user_already_exists_error(client: TestClient) -> None:
         f"{settings.API_V1_STR}/users/signup",
         json=data,
     )
-    assert r.status_code == 400
-    assert r.json()["detail"] == "The user with this email already exists in the system"
+    assert r.status_code == 409
+    assert r.json() == {
+        "error": "conflict",
+        "field": "email",
+        "message": "Already in use",
+    }
 
 
 def test_update_user(
diff --git a/frontend/src/routes/signup.tsx b/frontend/src/routes/signup.tsx
index 6e7890c485..7065ae5f9f 100644
--- a/frontend/src/routes/signup.tsx
+++ b/frontend/src/routes/signup.tsx
@@ -37,6 +37,7 @@ function SignUp() {
     register,
     handleSubmit,
     getValues,
+    setError,
     formState: { errors, isSubmitting },
   } = useForm<UserRegisterForm>({
     mode: "onBlur",
@@ -49,8 +50,21 @@ function SignUp() {
     },
   })
 
-  const onSubmit: SubmitHandler<UserRegisterForm> = (data) => {
-    signUpMutation.mutate(data)
+  const onSubmit: SubmitHandler<UserRegisterForm> = async (data) => {
+    try {
+      await signUpMutation.mutateAsync(data)
+    } catch (err: any) {
+      const status = err?.status
+      const body = err?.body as any
+      if (status === 409 && body?.error === "conflict") {
+        const field = (body?.field as "email" | "full_name" | "password") || "email"
+        const message = body?.message || "Already in use"
+        setError(field as any, { type: "server", message })
+        return
+      }
+      // rethrow to let the global onError handler show a toast
+      throw err
+    }
   }
 
   return (
diff --git a/frontend/tests/sign-up.spec.ts b/frontend/tests/sign-up.spec.ts
index 750edb1830..e4bed998db 100644
--- a/frontend/tests/sign-up.spec.ts
+++ b/frontend/tests/sign-up.spec.ts
@@ -95,9 +95,8 @@ test("Sign up with existing email", async ({ page }) => {
   await fillForm(page, fullName, email, password, password)
   await page.getByRole("button", { name: "Sign Up" }).click()
 
-  await page
-    .getByText("The user with this email already exists in the system")
-    .click()
+  // Expect inline error message
+  await expect(page.getByText("Already in use")).toBeVisible()
 })
 
 test("Sign up with weak password", async ({ page }) => {