diff --git a/app/web/routes.py b/app/web/routes.py
index 82e8f31..79eb6a7 100644
--- a/app/web/routes.py
+++ b/app/web/routes.py
@@ -24,6 +24,14 @@ def _cipher(request: Request) -> CredentialCipher:
     return CredentialCipher(request.app.state.settings.credential_encryption_key)
 
 
+def _require_non_empty(value: str, field_name: str) -> None:
+    if not value.strip():
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail=f"{field_name} must not be empty",
+        )
+
+
 @router.get("/login", response_class=HTMLResponse)
 def login_page(request: Request):
     return templates.TemplateResponse(request, "login.html", {"csrf_token": ensure_csrf_token(request)})
@@ -81,14 +89,18 @@ def create_user(
     request: Request,
     csrf_token: str = Form(...),
     name: str = Form(...),
-    mywhoosh_email: str = Form(...),
-    mywhoosh_password: str = Form(...),
-    garmin_email: str = Form(...),
-    garmin_password: str = Form(...),
+    mywhoosh_email: str = Form(""),
+    mywhoosh_password: str = Form(""),
+    garmin_email: str = Form(""),
+    garmin_password: str = Form(""),
     enabled: str | None = Form(None),
 ):
     require_admin(request)
     validate_csrf(request, csrf_token)
+    _require_non_empty(mywhoosh_email, "mywhoosh_email")
+    _require_non_empty(mywhoosh_password, "mywhoosh_password")
+    _require_non_empty(garmin_email, "garmin_email")
+    _require_non_empty(garmin_password, "garmin_password")
     form = UserFormData(
         name=name,
         mywhoosh_email=mywhoosh_email,
@@ -152,14 +164,16 @@ def update_user(
     user_id: int,
     csrf_token: str = Form(...),
     name: str = Form(...),
-    mywhoosh_email: str = Form(...),
+    mywhoosh_email: str = Form(""),
     mywhoosh_password: str = Form(""),
-    garmin_email: str = Form(...),
+    garmin_email: str = Form(""),
     garmin_password: str = Form(""),
     enabled: str | None = Form(None),
 ):
     require_admin(request)
     validate_csrf(request, csrf_token)
+    _require_non_empty(mywhoosh_email, "mywhoosh_email")
+    _require_non_empty(garmin_email, "garmin_email")
     form = UserFormData(
         name=name,
         mywhoosh_email=mywhoosh_email,
diff --git a/tests/web/test_users.py b/tests/web/test_users.py
index 82f72b6..ffc8763 100644
--- a/tests/web/test_users.py
+++ b/tests/web/test_users.py
@@ -234,3 +234,111 @@ def test_dashboard_lists_created_user_without_secrets(client: TestClient) -> Non
     assert "Max" in response.text
     assert "mw-secret" not in response.text
     assert "garmin-secret" not in response.text
+
+
+def _create_payload(**overrides: str) -> dict:
+    payload = {
+        "name": "Max",
+        "mywhoosh_email": "max@example.com",
+        "mywhoosh_password": "mw-secret",
+        "garmin_email": "max-garmin@example.com",
+        "garmin_password": "garmin-secret",
+        "enabled": "on",
+    }
+    payload.update(overrides)
+    return payload
+
+
+def test_create_user_rejects_empty_mywhoosh_email(client: TestClient) -> None:
+    login(client)
+    page = client.get("/users/new")
+    csrf = extract_csrf(page.text)
+    response = client.post(
+        "/users",
+        data={"csrf_token": csrf, **_create_payload(mywhoosh_email="")},
+    )
+    assert response.status_code == 400
+
+
+def test_create_user_rejects_empty_mywhoosh_password(client: TestClient) -> None:
+    login(client)
+    page = client.get("/users/new")
+    csrf = extract_csrf(page.text)
+    response = client.post(
+        "/users",
+        data={"csrf_token": csrf, **_create_payload(mywhoosh_password="")},
+    )
+    assert response.status_code == 400
+
+
+def test_create_user_rejects_empty_garmin_email(client: TestClient) -> None:
+    login(client)
+    page = client.get("/users/new")
+    csrf = extract_csrf(page.text)
+    response = client.post(
+        "/users",
+        data={"csrf_token": csrf, **_create_payload(garmin_email="")},
+    )
+    assert response.status_code == 400
+
+
+def test_create_user_rejects_empty_garmin_password(client: TestClient) -> None:
+    login(client)
+    page = client.get("/users/new")
+    csrf = extract_csrf(page.text)
+    response = client.post(
+        "/users",
+        data={"csrf_token": csrf, **_create_payload(garmin_password="")},
+    )
+    assert response.status_code == 400
+
+
+def test_create_user_rejects_whitespace_only_email(client: TestClient) -> None:
+    login(client)
+    page = client.get("/users/new")
+    csrf = extract_csrf(page.text)
+    response = client.post(
+        "/users",
+        data={"csrf_token": csrf, **_create_payload(mywhoosh_email="   ")},
+    )
+    assert response.status_code == 400
+
+
+def test_update_user_rejects_empty_mywhoosh_email(client: TestClient) -> None:
+    login(client)
+    user_id = create_user_via_http(client)
+    edit_page = client.get(f"/users/{user_id}/edit")
+    csrf = extract_csrf(edit_page.text)
+    response = client.post(
+        f"/users/{user_id}",
+        data={
+            "csrf_token": csrf,
+            "name": "Max",
+            "mywhoosh_email": "",
+            "mywhoosh_password": "",
+            "garmin_email": "max-garmin@example.com",
+            "garmin_password": "",
+            "enabled": "on",
+        },
+    )
+    assert response.status_code == 400
+
+
+def test_update_user_rejects_empty_garmin_email(client: TestClient) -> None:
+    login(client)
+    user_id = create_user_via_http(client)
+    edit_page = client.get(f"/users/{user_id}/edit")
+    csrf = extract_csrf(edit_page.text)
+    response = client.post(
+        f"/users/{user_id}",
+        data={
+            "csrf_token": csrf,
+            "name": "Max",
+            "mywhoosh_email": "max@example.com",
+            "mywhoosh_password": "",
+            "garmin_email": "",
+            "garmin_password": "",
+            "enabled": "on",
+        },
+    )
+    assert response.status_code == 400