diff --git a/api/custom_auth/oauth/serializers.py b/api/custom_auth/oauth/serializers.py
index 6a1e80ab90af..026db5fc93c5 100644
--- a/api/custom_auth/oauth/serializers.py
+++ b/api/custom_auth/oauth/serializers.py
@@ -13,6 +13,7 @@
 from users.models import SignUpType
 
 from ..constants import USER_REGISTRATION_WITHOUT_INVITE_ERROR_MESSAGE
+from ..serializers import InviteLinkValidationMixin
 from .github import GithubUser
 from .google import get_user_info
 
@@ -20,7 +21,7 @@
 UserModel = get_user_model()
 
 
-class OAuthLoginSerializer(serializers.Serializer):
+class OAuthLoginSerializer(InviteLinkValidationMixin, serializers.Serializer):
     access_token = serializers.CharField(
         required=True,
         help_text="Code or access token returned from the FE interaction with the third party login provider.",
diff --git a/api/custom_auth/serializers.py b/api/custom_auth/serializers.py
index 55bb43e595ae..9b56877e63a2 100644
--- a/api/custom_auth/serializers.py
+++ b/api/custom_auth/serializers.py
@@ -1,3 +1,5 @@
+from typing import Any
+
 from django.conf import settings
 from djoser.serializers import UserCreateSerializer
 from rest_framework import serializers
@@ -5,7 +7,7 @@
 from rest_framework.exceptions import PermissionDenied
 from rest_framework.validators import UniqueValidator
 
-from organisations.invites.models import Invite
+from organisations.invites.models import Invite, InviteLink
 from users.auth_type import AuthType
 from users.constants import DEFAULT_DELETE_ORPHAN_ORGANISATIONS_VALUE
 from users.models import FFAdminUser, SignUpType
@@ -23,7 +25,35 @@ class Meta:
         fields = ("key",)
 
 
-class CustomUserCreateSerializer(UserCreateSerializer):
+class InviteLinkValidationMixin:
+    invite_hash = serializers.CharField(required=False, write_only=True)
+
+    def validate(self, attrs: dict[str, Any]) -> dict[str, Any]:
+        attrs = super().validate(attrs)
+
+        if not settings.ALLOW_REGISTRATION_WITHOUT_INVITE:
+            self._validate_registration_invite(attrs)
+
+        return attrs
+
+    def _validate_registration_invite(self, attrs: dict[str, Any]) -> None:
+        valid = False
+
+        match attrs.get("sign_up_type"):
+            case SignUpType.INVITE_LINK.value:
+                valid = InviteLink.objects.filter(
+                    hash=self.initial_data.get("invite_hash")
+                ).exists()
+            case SignUpType.INVITE_EMAIL.value:
+                valid = Invite.objects.filter(
+                    email__iexact=attrs.get("email", "").lower()
+                ).exists()
+
+        if not valid:
+            raise PermissionDenied(USER_REGISTRATION_WITHOUT_INVITE_ERROR_MESSAGE)
+
+
+class CustomUserCreateSerializer(UserCreateSerializer, InviteLinkValidationMixin):
     key = serializers.SerializerMethodField()
 
     class Meta(UserCreateSerializer.Meta):
@@ -66,16 +96,6 @@ def get_key(instance):
         token, _ = Token.objects.get_or_create(user=instance)
         return token.key
 
-    def save(self, **kwargs):
-        if not (
-            settings.ALLOW_REGISTRATION_WITHOUT_INVITE
-            or self.validated_data.get("sign_up_type") == SignUpType.INVITE_LINK.value
-            or Invite.objects.filter(email=self.validated_data.get("email"))
-        ):
-            raise PermissionDenied(USER_REGISTRATION_WITHOUT_INVITE_ERROR_MESSAGE)
-
-        return super(CustomUserCreateSerializer, self).save(**kwargs)
-
 
 class CustomUserDelete(serializers.Serializer):
     current_password = serializers.CharField(
diff --git a/api/tests/unit/custom_auth/conftest.py b/api/tests/unit/custom_auth/conftest.py
new file mode 100644
index 000000000000..17d5f760c4c1
--- /dev/null
+++ b/api/tests/unit/custom_auth/conftest.py
@@ -0,0 +1,9 @@
+import pytest
+
+from organisations.invites.models import InviteLink
+from organisations.models import Organisation
+
+
+@pytest.fixture()
+def invite_link(organisation: Organisation) -> InviteLink:
+    return InviteLink.objects.create(organisation=organisation)
diff --git a/api/tests/unit/custom_auth/test_unit_custom_auth_serializer.py b/api/tests/unit/custom_auth/test_unit_custom_auth_serializer.py
index 00f099e1ace6..63731e0f7df3 100644
--- a/api/tests/unit/custom_auth/test_unit_custom_auth_serializer.py
+++ b/api/tests/unit/custom_auth/test_unit_custom_auth_serializer.py
@@ -1,7 +1,17 @@
+import pytest
 from django.test import RequestFactory
 from pytest_django.fixtures import SettingsWrapper
-
-from custom_auth.serializers import CustomUserCreateSerializer
+from rest_framework.exceptions import PermissionDenied
+from rest_framework.serializers import ModelSerializer
+
+from custom_auth.constants import (
+    USER_REGISTRATION_WITHOUT_INVITE_ERROR_MESSAGE,
+)
+from custom_auth.serializers import (
+    CustomUserCreateSerializer,
+    InviteLinkValidationMixin,
+)
+from organisations.invites.models import InviteLink
 from users.models import FFAdminUser, SignUpType
 
 user_dict = {
@@ -70,6 +80,7 @@ def test_CustomUserCreateSerializer_calls_is_authentication_method_valid_correct
 
 
 def test_CustomUserCreateSerializer_allows_registration_if_sign_up_type_is_invite_link(
+    invite_link: InviteLink,
     db: None,
     settings: SettingsWrapper,
     rf: RequestFactory,
@@ -80,6 +91,7 @@ def test_CustomUserCreateSerializer_allows_registration_if_sign_up_type_is_invit
     data = {
         **user_dict,
         "sign_up_type": SignUpType.INVITE_LINK.value,
+        "invite_hash": invite_link.hash,
     }
 
     serializer = CustomUserCreateSerializer(
@@ -92,3 +104,52 @@ def test_CustomUserCreateSerializer_allows_registration_if_sign_up_type_is_invit
 
     # Then
     assert user
+
+
+def test_invite_link_validation_mixin_validate_fails_if_invite_link_hash_not_provided(
+    settings: SettingsWrapper,
+    db: None,
+) -> None:
+    # Given
+    settings.ALLOW_REGISTRATION_WITHOUT_INVITE = False
+
+    class TestSerializer(InviteLinkValidationMixin, ModelSerializer):
+        class Meta:
+            model = FFAdminUser
+            fields = ("sign_up_type",)
+
+    serializer = TestSerializer(data={"sign_up_type": SignUpType.INVITE_LINK.value})
+
+    # When
+    with pytest.raises(PermissionDenied) as exc_info:
+        serializer.is_valid(raise_exception=True)
+
+    # Then
+    assert exc_info.value.detail == USER_REGISTRATION_WITHOUT_INVITE_ERROR_MESSAGE
+
+
+def test_invite_link_validation_mixin_validate_fails_if_invite_link_hash_not_valid(
+    invite_link: InviteLink,
+    settings: SettingsWrapper,
+) -> None:
+    # Given
+    settings.ALLOW_REGISTRATION_WITHOUT_INVITE = False
+
+    class TestSerializer(InviteLinkValidationMixin, ModelSerializer):
+        class Meta:
+            model = FFAdminUser
+            fields = ("sign_up_type",)
+
+    serializer = TestSerializer(
+        data={
+            "sign_up_type": SignUpType.INVITE_LINK.value,
+            "invite_hash": "invalid-hash",
+        }
+    )
+
+    # When
+    with pytest.raises(PermissionDenied) as exc_info:
+        serializer.is_valid(raise_exception=True)
+
+    # Then
+    assert exc_info.value.detail == USER_REGISTRATION_WITHOUT_INVITE_ERROR_MESSAGE