diff --git a/src/registrar/forms/portfolio.py b/src/registrar/forms/portfolio.py
index 5aef09389..b83e718cb 100644
--- a/src/registrar/forms/portfolio.py
+++ b/src/registrar/forms/portfolio.py
@@ -445,3 +445,28 @@ class PortfolioNewMemberForm(BasePortfolioMemberForm):
     class Meta:
         model = PortfolioInvitation
         fields = ["portfolio", "email", "roles", "additional_permissions"]
+
+    def _post_clean(self):
+        """
+        Override _post_clean to customize model validation errors.
+        This runs after form clean is complete, but before the errors are displayed.
+        """
+        try:
+            super()._post_clean()
+            self.instance.clean()
+        except forms.ValidationError as e:
+            override_error = False
+            if hasattr(e, "code"):
+                field = "email" if "email" in self.fields else None
+                if e.code == "has_existing_permissions":
+                    self.add_error(field, f"{self.instance.email} is already a member of another .gov organization.")
+                    override_error = True
+                elif e.code == "has_existing_invitations":
+                    self.add_error(
+                        field, f"{self.instance.email} has already been invited to another .gov organization."
+                    )
+                    override_error = True
+
+            # Errors denoted as "__all__" are special error types reserved for the model level clean function
+            if override_error and "__all__" in self._errors:
+                del self._errors["__all__"]
diff --git a/src/registrar/models/utility/portfolio_helper.py b/src/registrar/models/utility/portfolio_helper.py
index 03dca897f..009ea3c26 100644
--- a/src/registrar/models/utility/portfolio_helper.py
+++ b/src/registrar/models/utility/portfolio_helper.py
@@ -292,7 +292,8 @@ def validate_user_portfolio_permission(user_portfolio_permission):
         if existing_permissions.exists():
             raise ValidationError(
                 "This user is already assigned to a portfolio. "
-                "Based on current waffle flag settings, users cannot be assigned to multiple portfolios."
+                "Based on current waffle flag settings, users cannot be assigned to multiple portfolios.",
+                code="has_existing_permissions",
             )
 
         existing_invitations = PortfolioInvitation.objects.filter(email=user_portfolio_permission.user.email).exclude(
@@ -302,7 +303,8 @@ def validate_user_portfolio_permission(user_portfolio_permission):
         if existing_invitations.exists():
             raise ValidationError(
                 "This user is already assigned to a portfolio invitation. "
-                "Based on current waffle flag settings, users cannot be assigned to multiple portfolios."
+                "Based on current waffle flag settings, users cannot be assigned to multiple portfolios.",
+                code="has_existing_invitations",
             )
 
 
@@ -350,6 +352,7 @@ def validate_portfolio_invitation(portfolio_invitation):
 
     # == Validate the multiple_porfolios flag. == #
     user = User.objects.filter(email=portfolio_invitation.email).first()
+
     # If user returns None, then we check for global assignment of multiple_portfolios.
     # Otherwise we just check on the user.
     if not flag_is_active_for_user(user, "multiple_portfolios"):
@@ -362,13 +365,15 @@ def validate_portfolio_invitation(portfolio_invitation):
         if existing_permissions.exists():
             raise ValidationError(
                 "This user is already assigned to a portfolio. "
-                "Based on current waffle flag settings, users cannot be assigned to multiple portfolios."
+                "Based on current waffle flag settings, users cannot be assigned to multiple portfolios.",
+                code="has_existing_permissions",
             )
 
         if existing_invitations.exists():
             raise ValidationError(
                 "This user is already assigned to a portfolio invitation. "
-                "Based on current waffle flag settings, users cannot be assigned to multiple portfolios."
+                "Based on current waffle flag settings, users cannot be assigned to multiple portfolios.",
+                code="has_existing_invitations",
             )
 
 
diff --git a/src/registrar/tests/test_views_portfolio.py b/src/registrar/tests/test_views_portfolio.py
index 64bcb50a8..92e053576 100644
--- a/src/registrar/tests/test_views_portfolio.py
+++ b/src/registrar/tests/test_views_portfolio.py
@@ -3873,11 +3873,7 @@ class TestPortfolioInviteNewMemberView(MockEppLib, WebTest):
         # verify messages
         self.assertContains(
             response,
-            (
-                "This user is already assigned to a portfolio invitation. "
-                "Based on current waffle flag settings, users cannot be assigned "
-                "to multiple portfolios."
-            ),
+            f"{self.invited_member_email} has already been invited to another .gov organization.",
         )
 
         # Validate Database has not changed
@@ -3915,11 +3911,7 @@ class TestPortfolioInviteNewMemberView(MockEppLib, WebTest):
         # Verify messages
         self.assertContains(
             response,
-            (
-                "This user is already assigned to a portfolio. "
-                "Based on current waffle flag settings, users cannot be "
-                "assigned to multiple portfolios."
-            ),
+            f"{self.user.email} is already a member of another .gov organization.",
         )
 
         # Validate Database has not changed