diff --git a/api_schemas/user_schemas.py b/api_schemas/user_schemas.py
index 04b8d3d..c765755 100644
--- a/api_schemas/user_schemas.py
+++ b/api_schemas/user_schemas.py
@@ -1,5 +1,6 @@
 from typing import TYPE_CHECKING, Annotated, Literal
-from pydantic import StringConstraints
+from pydantic import StringConstraints, field_validator
+from helpers.check_stil_id import check_stil_id
 from fastapi_users_pelicanq import schemas as fastapi_users_schemas
 from api_schemas.post_schemas import PostRead
 from helpers.constants import MAX_FIRST_NAME_LEN, MAX_LAST_NAME_LEN
@@ -11,6 +12,19 @@
     from api_schemas.group_schema import GroupRead
 
 
+class StilIdValidationMixin:
+    stil_id: str | None = None
+
+    @field_validator("stil_id", mode="after")
+    @classmethod
+    def validate_stil_id(cls, value: str | None) -> str | None:
+        if value is None:
+            return None
+        if not check_stil_id(value):
+            raise ValueError("Invalid stil-id")
+        return value
+
+
 class _UserEventRead(BaseSchema):
     id: int
 
@@ -118,25 +132,25 @@ class UserInGroupRead(BaseSchema):
 
 
 # fastapi-users will take all fields on this model and feed into the user constructor User_DB(...) when /auth/register route is called
-class UserCreate(fastapi_users_schemas.BaseUserCreate, BaseSchema):
+class UserCreate(StilIdValidationMixin, fastapi_users_schemas.BaseUserCreate, BaseSchema):
     first_name: Annotated[str, StringConstraints(max_length=MAX_FIRST_NAME_LEN)]
     last_name: Annotated[str, StringConstraints(max_length=MAX_LAST_NAME_LEN)]
     telephone_number: PhoneNumber
     start_year: int | None = None
-    pass
+    # stil_id is here, but inherited from StilIdValidationMixin
 
 
-class UserUpdate(BaseSchema):
+class UserUpdate(StilIdValidationMixin, BaseSchema):
     first_name: str | None = None
     last_name: str | None = None
     start_year: int | None = None
     program: PROGRAM_TYPE | None = None
     notifications: bool | None = None
-    stil_id: str | None = None
     standard_food_preferences: list[str] | None = None
     other_food_preferences: str | None = None
     telephone_number: PhoneNumber | None = None
     moose_game_name: str | None = None
+    # stil_id is here, but inherited from StilIdValidationMixin
 
 
 class UpdateUserMember(BaseSchema):
diff --git a/helpers/check_stil_id.py b/helpers/check_stil_id.py
new file mode 100644
index 0000000..3d71214
--- /dev/null
+++ b/helpers/check_stil_id.py
@@ -0,0 +1,8 @@
+import re
+
+
+def check_stil_id(s: str) -> bool:
+    if not len(s) == 10:
+        return False
+    pattern = r"^[a-z]{2}\d{4}[a-z]{2}-s$"
+    return bool(re.fullmatch(pattern, s))
diff --git a/services/user.py b/services/user.py
index 21759b0..ea7911b 100644
--- a/services/user.py
+++ b/services/user.py
@@ -4,16 +4,9 @@
 from db_models.user_model import User_DB
 from fastapi import HTTPException, status
 from sqlalchemy.exc import DataError, NoResultFound, MultipleResultsFound
-import re
 from helpers.types import FOOD_PREFERENCES
 from db_models.post_model import Post_DB
-
-
-def check_stil_id(s: str) -> bool:
-    if not len(s) == 10:
-        return False
-    pattern = r"^[a-z]{2}\d{4}[a-z]{2}-s$"
-    return bool(re.fullmatch(pattern, s))
+from helpers.check_stil_id import check_stil_id
 
 
 def condition(model, asset):
diff --git a/tests/test_users.py b/tests/test_users.py
index 4f6e02a..9118a6b 100644
--- a/tests/test_users.py
+++ b/tests/test_users.py
@@ -15,6 +15,19 @@ def test_register_user(client, user1_data):
     assert "id" in data
 
 
+def test_register_user_invalid_stil_id(client):
+    """Test user registration with valid data except for an invalid stil_id"""
+    user_data = user_data_factory(
+        email="test1@example.com",
+        last_name="User1",
+        program="F",
+        telephone_number="+46701234567",
+        stil_id="invalid_stil_id",
+    )
+    response = client.post("/auth/register", json=user_data)
+    assert response.status_code == status.HTTP_422_UNPROCESSABLE_ENTITY
+
+
 def test_register_duplicate_user(client, user1_data):
     """Test registration with duplicate email fails"""
     # Register first user