diff --git a/awx/api/serializers.py b/awx/api/serializers.py
index 1fb9a43f72..1281351ade 100644
--- a/awx/api/serializers.py
+++ b/awx/api/serializers.py
@@ -4694,6 +4694,8 @@ class InstanceGroupSerializer(BaseSerializer):
                 raise serializers.ValidationError(_('{} is not a valid hostname of an existing instance.').format(instance_name))
             if Instance.objects.get(hostname=instance_name).is_isolated():
                 raise serializers.ValidationError(_('Isolated instances may not be added or removed from instances groups via the API.'))
+            if self.instance and self.instance.controller_id is not None:
+                raise serializers.ValidationError(_('Isolated instance group membership may not be managed via the API.'))
         return value
 
     def validate_name(self, value):
diff --git a/awx/api/views.py b/awx/api/views.py
index c99821e5b8..64e326c4eb 100644
--- a/awx/api/views.py
+++ b/awx/api/views.py
@@ -192,6 +192,10 @@ class InstanceGroupMembershipMixin(object):
     def is_valid_relation(self, parent, sub, created=False):
         if sub.is_isolated():
             return {'error': _('Isolated instances may not be added or removed from instances groups via the API.')}
+        if self.parent_model is InstanceGroup:
+            ig_obj = self.get_parent_object()
+            if ig_obj.controller_id is not None:
+                return {'error': _('Isolated instance group membership may not be managed via the API.')}
         return None
 
     def unattach_validate(self, request):
diff --git a/awx/main/tests/functional/api/test_instance_group.py b/awx/main/tests/functional/api/test_instance_group.py
index c9a0ad4a28..939db63dc2 100644
--- a/awx/main/tests/functional/api/test_instance_group.py
+++ b/awx/main/tests/functional/api/test_instance_group.py
@@ -21,6 +21,11 @@ def instance():
     return instance
 
 
+@pytest.fixture
+def non_iso_instance():
+    return Instance.objects.create(hostname='iamnotanisolatedinstance')
+
+
 @pytest.fixture
 def instance_group(job_factory):
     ig = InstanceGroup(name="east")
@@ -138,7 +143,7 @@ def test_prevent_isolated_instance_added_to_non_isolated_instance_group_via_poli
     url = reverse("api:instance_group_detail", kwargs={'pk': instance_group.pk})
 
     assert True is instance.is_isolated()
-    resp = patch(url, {'policy_instance_list': [instance.hostname]}, admin)
+    resp = patch(url, {'policy_instance_list': [instance.hostname]}, user=admin, expect=400)
     assert [u"Isolated instances may not be added or removed from instances groups via the API."] == resp.data['policy_instance_list']
     assert instance_group.policy_instance_list == []
 
@@ -150,3 +155,24 @@ def test_prevent_isolated_instance_removal_from_isolated_instance_group(post, ad
     assert True is instance.is_isolated()
     resp = post(url, {'disassociate': True, 'id': instance.id}, admin, expect=400)
     assert u"Isolated instances may not be added or removed from instances groups via the API." == resp.data['error']
+
+
+@pytest.mark.django_db
+def test_prevent_non_isolated_instance_added_to_isolated_instance_group(
+        post, admin, non_iso_instance, isolated_instance_group):
+    url = reverse("api:instance_group_instance_list", kwargs={'pk': isolated_instance_group.pk})
+
+    assert False is non_iso_instance.is_isolated()
+    resp = post(url, {'associate': True, 'id': non_iso_instance.id}, admin, expect=400)
+    assert u"Isolated instance group membership may not be managed via the API." == resp.data['error']
+
+
+@pytest.mark.django_db
+def test_prevent_non_isolated_instance_added_to_isolated_instance_group_via_policy_list(
+        patch, admin, non_iso_instance, isolated_instance_group):
+    url = reverse("api:instance_group_detail", kwargs={'pk': isolated_instance_group.pk})
+
+    assert False is non_iso_instance.is_isolated()
+    resp = patch(url, {'policy_instance_list': [non_iso_instance.hostname]}, user=admin, expect=400)
+    assert [u"Isolated instance group membership may not be managed via the API."] == resp.data['policy_instance_list']
+    assert isolated_instance_group.policy_instance_list == []