diff --git a/netbox/dcim/api/serializers_/device_components.py b/netbox/dcim/api/serializers_/device_components.py
index f26e81aa1..16b68671b 100644
--- a/netbox/dcim/api/serializers_/device_components.py
+++ b/netbox/dcim/api/serializers_/device_components.py
@@ -244,7 +244,7 @@ class InterfaceSerializer(NetBoxModelSerializer, CabledObjectSerializer, Connect
             tagged_vlans = []
 
             if self.instance.pk and 'mode' in data.keys():
-                mode = data.get('mode') if 'mode' in self.data.keys() else self.instance.get('mode')
+                mode = data.get('mode') if 'mode' in data.keys() else self.instance.get('mode')
             elif 'mode' in data.keys():
                 mode = data.get('mode')
 
@@ -254,9 +254,15 @@ class InterfaceSerializer(NetBoxModelSerializer, CabledObjectSerializer, Connect
             elif 'tagged_vlans' in data.keys():
                 tagged_vlans = data.get('tagged_vlans')
 
+            if self.instance.pk and 'untagged_vlan' in data.keys():
+                untagged_vlan = data.get('untagged_vlan') if 'untagged_vlan' in data.keys() else \
+                    self.instance.untagged_vlan
+            elif 'untagged_vlan' in data.keys():
+                untagged_vlan = data.get('untagged_vlan')
+
             if mode != InterfaceModeChoices.MODE_TAGGED and tagged_vlans:
                 raise serializers.ValidationError({
-                    'tagged_vlans': "Interface mode does not support including tagged vlans"
+                    'tagged_vlans': _("Interface mode does not support including tagged vlans")
                 })
 
             # Validate many-to-many VLAN assignments
diff --git a/netbox/dcim/forms/model_forms.py b/netbox/dcim/forms/model_forms.py
index a03175725..b26247a94 100644
--- a/netbox/dcim/forms/model_forms.py
+++ b/netbox/dcim/forms/model_forms.py
@@ -1366,6 +1366,7 @@ class InterfaceForm(InterfaceCommonForm, ModularDeviceComponentForm):
     def clean(self):
         mode = None
         tagged_vlans = []
+        untagged_vlan = None
 
         if self.instance.pk and 'mode' in self.cleaned_data.keys():
             mode = self.cleaned_data.get('mode') if 'mode' in self.cleaned_data.keys() else self.instance.get('mode')
@@ -1378,9 +1379,15 @@ class InterfaceForm(InterfaceCommonForm, ModularDeviceComponentForm):
         elif 'tagged_vlans' in self.cleaned_data.keys():
             tagged_vlans = self.cleaned_data.get('tagged_vlans')
 
+        if self.instance.pk and 'untagged_vlan' in self.cleaned_data.keys():
+            untagged_vlan = self.cleaned_data.get('untagged_vlan') if 'untagged_vlan' in self.cleaned_data.keys() else\
+                self.instance.untagged_vlan
+        elif 'untagged_vlan' in self.cleaned_data.keys():
+            untagged_vlan = self.cleaned_data.get('untagged_vlan')
+
         if mode != InterfaceModeChoices.MODE_TAGGED and tagged_vlans:
             raise forms.ValidationError({
-                'tagged_vlans': "Interface mode does not support including tagged vlans"
+                'tagged_vlans': _("Interface mode does not support including tagged vlans")
             })
 
 
diff --git a/netbox/dcim/models/device_components.py b/netbox/dcim/models/device_components.py
index 101a1a459..f38343f5d 100644
--- a/netbox/dcim/models/device_components.py
+++ b/netbox/dcim/models/device_components.py
@@ -894,7 +894,7 @@ class Interface(ModularComponentModel, BaseInterface, CabledObjectModel, PathEnd
                 raise ValidationError({'rf_channel_width': _("Cannot specify custom width with channel selected.")})
 
         # VLAN validation
-        if self.mode is None and self.untagged_vlan:
+        if not self.mode and self.untagged_vlan:
             raise ValidationError({'untagged_vlan': _("Interface mode does not support including an untagged vlan.")})
 
         # Validate untagged VLAN