diff --git a/netbox/extras/api/customfields.py b/netbox/extras/api/customfields.py
index a053642db..393e4545f 100644
--- a/netbox/extras/api/customfields.py
+++ b/netbox/extras/api/customfields.py
@@ -1,10 +1,9 @@
 from datetime import datetime
 
 from django.contrib.contenttypes.models import ContentType
-from django.core.exceptions import ObjectDoesNotExist
 from rest_framework import serializers
 from rest_framework.exceptions import ValidationError
-from rest_framework.fields import CreateOnlyDefault
+from rest_framework.fields import CreateOnlyDefault, Field
 
 from extras.choices import *
 from extras.models import CustomField
@@ -46,12 +45,18 @@ class CustomFieldDefaultValues:
         return value
 
 
-class CustomFieldsSerializer(serializers.BaseSerializer):
+class CustomFieldsDataField(Field):
 
     def to_representation(self, obj):
-        return obj
+        content_type = ContentType.objects.get_for_model(self.parent.Meta.model)
+        custom_fields = CustomField.objects.filter(obj_type=content_type)
+
+        return {cf.name: obj.get(cf.name) for cf in custom_fields}
 
     def to_internal_value(self, data):
+        # If updating an existing instance, start with existing custom_field_data
+        if self.parent.instance:
+            data = {**self.parent.instance.custom_field_data, **data}
 
         content_type = ContentType.objects.get_for_model(self.parent.Meta.model)
         custom_fields = {
@@ -111,9 +116,8 @@ class CustomFieldModelSerializer(ValidatedModelSerializer):
     """
     Extends ModelSerializer to render any CustomFields and their values associated with an object.
     """
-    custom_fields = CustomFieldsSerializer(
+    custom_fields = CustomFieldsDataField(
         source='custom_field_data',
-        required=False,
         default=CreateOnlyDefault(CustomFieldDefaultValues())
     )
 
diff --git a/netbox/extras/tests/test_customfields.py b/netbox/extras/tests/test_customfields.py
index 675248a3b..31d3c2be9 100644
--- a/netbox/extras/tests/test_customfields.py
+++ b/netbox/extras/tests/test_customfields.py
@@ -393,19 +393,17 @@ class CustomFieldAPITest(APITestCase):
 
         # Validate response data
         response_cf = response.data['custom_fields']
-        data_cf = data['custom_fields']
-        self.assertEqual(response_cf['text_field'], data_cf['text_field'])
-        self.assertEqual(response_cf['number_field'], data_cf['number_field'])
-        # TODO: Non-updated fields are missing from the response data
-        # self.assertEqual(response_cf['boolean_field'], site2_original_cfvs['boolean_field'])
-        # self.assertEqual(response_cf['date_field'], site2_original_cfvs['date_field'])
-        # self.assertEqual(response_cf['url_field'], site2_original_cfvs['url_field'])
-        # self.assertEqual(response_cf['choice_field'], site2_original_cfvs['choice_field'].value)
+        self.assertEqual(response_cf['text_field'], data['custom_fields']['text_field'])
+        self.assertEqual(response_cf['number_field'], data['custom_fields']['number_field'])
+        self.assertEqual(response_cf['boolean_field'], original_cfvs['boolean_field'])
+        self.assertEqual(response_cf['date_field'], original_cfvs['date_field'])
+        self.assertEqual(response_cf['url_field'], original_cfvs['url_field'])
+        self.assertEqual(response_cf['choice_field'], original_cfvs['choice_field'])
 
         # Validate database data
         site.refresh_from_db()
-        self.assertEqual(site.custom_field_data['text_field'], data_cf['text_field'])
-        self.assertEqual(site.custom_field_data['number_field'], data_cf['number_field'])
+        self.assertEqual(site.custom_field_data['text_field'], data['custom_fields']['text_field'])
+        self.assertEqual(site.custom_field_data['number_field'], data['custom_fields']['number_field'])
         self.assertEqual(site.custom_field_data['boolean_field'], original_cfvs['boolean_field'])
         self.assertEqual(site.custom_field_data['date_field'], original_cfvs['date_field'])
         self.assertEqual(site.custom_field_data['url_field'], original_cfvs['url_field'])
@@ -456,7 +454,7 @@ class CustomFieldImportTest(TestCase):
         self.assertEqual(site1.custom_field_data['boolean'], True)
         self.assertEqual(site1.custom_field_data['date'], '2020-01-01')
         self.assertEqual(site1.custom_field_data['url'], 'http://example.com/1')
-        self.assertEqual(site1.custom_field_data['select'].value, 'Choice A')
+        self.assertEqual(site1.custom_field_data['select'], 'Choice A')
 
         # Validate data for site 2
         site2 = Site.objects.get(name='Site 2')
@@ -466,7 +464,7 @@ class CustomFieldImportTest(TestCase):
         self.assertEqual(site2.custom_field_data['boolean'], False)
         self.assertEqual(site2.custom_field_data['date'], '2020-01-02')
         self.assertEqual(site2.custom_field_data['url'], 'http://example.com/2')
-        self.assertEqual(site2.custom_field_data['select'].value, 'Choice B')
+        self.assertEqual(site2.custom_field_data['select'], 'Choice B')
 
         # No CustomFieldValues should be created for site 3
         site3 = Site.objects.get(name='Site 3')