diff --git a/src/registrar/models/user.py b/src/registrar/models/user.py
index 29e0bc90d..21e7e196f 100644
--- a/src/registrar/models/user.py
+++ b/src/registrar/models/user.py
@@ -212,11 +212,11 @@ class User(AbstractUser):
         """Extends clean method to perform additional validation, which can raise errors in django admin."""
         super().clean()
 
-        portfolio_perms = self.portfolio_permissions.filter(portfolio=self.last_selected_portfolio).first()
-        if self.last_selected_portfolio is None and portfolio_perms._get_portfolio_permissions():
+        portfolio_perm = self.portfolio_permissions.filter(portfolio=self.last_selected_portfolio, user=self).first()
+        if self.last_selected_portfolio is None and portfolio_perm._get_portfolio_permissions():
             raise ValidationError("When portfolio roles or additional permissions are assigned, portfolio is required.")
 
-        if self.last_selected_portfolio is not None and not portfolio_perms._get_portfolio_permissions():
+        if self.last_selected_portfolio is not None and not portfolio_perm._get_portfolio_permissions():
             raise ValidationError("When portfolio is assigned, portfolio roles or additional permissions are required.")
 
     def _has_portfolio_permission(self, portfolio_permission):
@@ -225,7 +225,7 @@ class User(AbstractUser):
         if not self.last_selected_portfolio:
             return False
 
-        portfolio_perms = self.portfolio_permissions.filter(portfolio=self.last_selected_portfolio).first()
+        portfolio_perms = self.portfolio_permissions.filter(portfolio=self.last_selected_portfolio, user=self).first()
         if not portfolio_perms:
             return False
 
diff --git a/src/registrar/models/user_portfolio_permission.py b/src/registrar/models/user_portfolio_permission.py
index d88c2b4f9..44d1199fc 100644
--- a/src/registrar/models/user_portfolio_permission.py
+++ b/src/registrar/models/user_portfolio_permission.py
@@ -1,4 +1,6 @@
 from django.db import models
+from django.forms import ValidationError
+from waffle import flag_is_active
 from registrar.models.utility.portfolio_helper import UserPortfolioPermissionChoices, UserPortfolioRoleChoices
 from .utility.time_stamped_model import TimeStampedModel
 from django.contrib.postgres.fields import ArrayField
@@ -93,3 +95,12 @@ class UserPortfolioPermission(TimeStampedModel):
             portfolio_permissions.update(self.portfolio_additional_permissions)
 
         return list(portfolio_permissions)
+    
+    def clean(self):
+        """Extends clean method to perform additional validation, which can raise errors in django admin."""
+        super().clean()
+
+        if not flag_is_active(None, "multiple_portfolios") and self.pk is None:
+            existing_permissions = UserPortfolioPermission.objects.filter(user=self.user)
+            if existing_permissions.exists():
+                raise ValidationError("Only one portfolio permission is allowed per user when multiple portfolios are disabled.")