diff --git a/netbox/extras/tests/test_api.py b/netbox/extras/tests/test_api.py
index 255457f21..52c988ed3 100644
--- a/netbox/extras/tests/test_api.py
+++ b/netbox/extras/tests/test_api.py
@@ -2,16 +2,19 @@ import datetime
 
 from django.contrib.auth import get_user_model
 from django.contrib.contenttypes.models import ContentType
+from django.test import override_settings
 from django.urls import reverse
 from django.utils.timezone import make_aware
 from rest_framework import status
 
+from circuits.api.serializers import ProviderSerializer
 from core.choices import ManagedFileRootPathChoices
 from dcim.models import Device, DeviceRole, DeviceType, Manufacturer, Rack, Location, RackRole, Site
 from extras.models import *
 from extras.reports import Report
 from extras.scripts import BooleanVar, IntegerVar, Script, StringVar
-from utilities.testing import APITestCase, APIViewTestCases
+from ipam.models import ASN, RIR
+from utilities.testing import APITestCase, APIViewTestCases, create_tags
 
 
 User = get_user_model()
@@ -829,3 +832,53 @@ class ContentTypeTest(APITestCase):
 
         url = reverse('extras-api:contenttype-detail', kwargs={'pk': contenttype.pk})
         self.assertHttpStatus(self.client.get(url, **self.header), status.HTTP_200_OK)
+
+
+class CustomValidationTest(APITestCase):
+
+    @override_settings(CUSTOM_VALIDATORS={
+        'circuits.provider': [
+            {'tags': {'required': True}}
+        ]
+    })
+    def test_tags_validation(self):
+        """
+        Check that custom validation rules work for tag assignment.
+        """
+        data = {
+            'name': 'Provider 1',
+            'slug': 'provider-1',
+        }
+        serializer = ProviderSerializer(data=data)
+        self.assertFalse(serializer.is_valid())
+
+        tags = create_tags('Tag1', 'Tag2', 'Tag3')
+        data['tags'] = [tag.pk for tag in tags]
+        serializer = ProviderSerializer(data=data)
+        self.assertTrue(serializer.is_valid())
+
+    @override_settings(CUSTOM_VALIDATORS={
+        'circuits.provider': [
+            {'asns': {'required': True}}
+        ]
+    })
+    def test_m2m_validation(self):
+        """
+        Check that custom validation rules work for many-to-many fields.
+        """
+        data = {
+            'name': 'Provider 1',
+            'slug': 'provider-1',
+        }
+        serializer = ProviderSerializer(data=data)
+        self.assertFalse(serializer.is_valid())
+
+        rir = RIR.objects.create(name='RIR 1', slug='rir-1')
+        asns = ASN.objects.bulk_create((
+            ASN(rir=rir, asn=65001),
+            ASN(rir=rir, asn=65002),
+            ASN(rir=rir, asn=65003),
+        ))
+        data['asns'] = [asn.pk for asn in asns]
+        serializer = ProviderSerializer(data=data)
+        self.assertTrue(serializer.is_valid())
diff --git a/netbox/extras/tests/test_forms.py b/netbox/extras/tests/test_forms.py
index 9c22bf83c..ac94dba64 100644
--- a/netbox/extras/tests/test_forms.py
+++ b/netbox/extras/tests/test_forms.py
@@ -1,11 +1,14 @@
 from django.contrib.contenttypes.models import ContentType
-from django.test import TestCase
+from django.test import TestCase, override_settings
 
+from circuits.forms import ProviderForm
 from dcim.forms import SiteForm
 from dcim.models import Site
 from extras.choices import CustomFieldTypeChoices
 from extras.forms import SavedFilterForm
 from extras.models import CustomField, CustomFieldChoiceSet
+from ipam.models import ASN, RIR
+from utilities.testing import create_tags
 
 
 class CustomFieldModelFormTest(TestCase):
@@ -109,3 +112,53 @@ class SavedFilterFormTest(TestCase):
         })
         self.assertTrue(form.is_valid())
         form.save()
+
+
+class CustomValidationTest(TestCase):
+
+    @override_settings(CUSTOM_VALIDATORS={
+        'circuits.provider': [
+            {'tags': {'required': True}}
+        ]
+    })
+    def test_tags_validation(self):
+        """
+        Check that custom validation rules work for tag assignment.
+        """
+        data = {
+            'name': 'Provider 1',
+            'slug': 'provider-1',
+        }
+        form = ProviderForm(data)
+        self.assertFalse(form.is_valid())
+
+        tags = create_tags('Tag1', 'Tag2', 'Tag3')
+        data['tags'] = [tag.pk for tag in tags]
+        form = ProviderForm(data)
+        self.assertTrue(form.is_valid())
+
+    @override_settings(CUSTOM_VALIDATORS={
+        'circuits.provider': [
+            {'asns': {'required': True}}
+        ]
+    })
+    def test_m2m_validation(self):
+        """
+        Check that custom validation rules work for many-to-many fields.
+        """
+        data = {
+            'name': 'Provider 1',
+            'slug': 'provider-1',
+        }
+        form = ProviderForm(data)
+        self.assertFalse(form.is_valid())
+
+        rir = RIR.objects.create(name='RIR 1', slug='rir-1')
+        asns = ASN.objects.bulk_create((
+            ASN(rir=rir, asn=65001),
+            ASN(rir=rir, asn=65002),
+            ASN(rir=rir, asn=65003),
+        ))
+        data['asns'] = [asn.pk for asn in asns]
+        form = ProviderForm(data)
+        self.assertTrue(form.is_valid())
diff --git a/netbox/extras/validators.py b/netbox/extras/validators.py
index 686c9b032..82e11f3f9 100644
--- a/netbox/extras/validators.py
+++ b/netbox/extras/validators.py
@@ -1,5 +1,6 @@
-from django.core.exceptions import ValidationError
 from django.core import validators
+from django.core.exceptions import ValidationError
+from django.utils.translation import gettext_lazy as _
 
 # NOTE: As this module may be imported by configuration.py, we cannot import
 # anything from NetBox itself.
@@ -66,8 +67,7 @@ class CustomValidator:
     def __call__(self, instance):
         # Validate instance attributes per validation rules
         for attr_name, rules in self.validation_rules.items():
-            assert hasattr(instance, attr_name), f"Invalid attribute '{attr_name}' for {instance.__class__.__name__}"
-            attr = getattr(instance, attr_name)
+            attr = self._getattr(instance, attr_name)
             for descriptor, value in rules.items():
                 validator = self.get_validator(descriptor, value)
                 try:
@@ -79,6 +79,28 @@ class CustomValidator:
         # Execute custom validation logic (if any)
         self.validate(instance)
 
+    @staticmethod
+    def _getattr(instance, name):
+        # Attempt to resolve many-to-many fields to their stored values
+        m2m_fields = [f.name for f in instance._meta.local_many_to_many]
+        if name in m2m_fields:
+            if name in instance._m2m_values:
+                return instance._m2m_values[name]
+            elif instance.pk:
+                # TODO: Handle invalid attrs
+                return list(getattr(instance, name).all())
+            else:
+                return []
+
+        # Raise a ValidationError for unknown attributes
+        elif not hasattr(instance, name):
+            raise ValidationError(_('Invalid attribute "{name}" for {model}').format(
+                name=name,
+                model=instance.__class__.__name__
+            ))
+
+        return getattr(instance, name)
+
     def get_validator(self, descriptor, value):
         """
         Instantiate and return the appropriate validator based on the descriptor given. For
diff --git a/netbox/netbox/api/serializers/base.py b/netbox/netbox/api/serializers/base.py
index 5ee74bf8c..b2809cf3d 100644
--- a/netbox/netbox/api/serializers/base.py
+++ b/netbox/netbox/api/serializers/base.py
@@ -24,15 +24,15 @@ class ValidatedModelSerializer(BaseModelSerializer):
     """
     def validate(self, data):
 
-        # Remove custom fields data and tags (if any) prior to model validation
+        # Remove custom fields data (if any) prior to model validation
         attrs = data.copy()
         attrs.pop('custom_fields', None)
-        attrs.pop('tags', None)
 
         # Skip ManyToManyFields
-        for field in self.Meta.model._meta.get_fields():
-            if isinstance(field, ManyToManyField):
-                attrs.pop(field.name, None)
+        m2m_values = {}
+        for field in self.Meta.model._meta.local_many_to_many:
+            if field.name in attrs:
+                m2m_values[field.name] = attrs.pop(field.name)
 
         # Run clean() on an instance of the model
         if self.instance is None:
@@ -41,6 +41,7 @@ class ValidatedModelSerializer(BaseModelSerializer):
             instance = self.instance
             for k, v in attrs.items():
                 setattr(instance, k, v)
+        instance._m2m_values = m2m_values
         instance.full_clean()
 
         return data
diff --git a/netbox/netbox/forms/base.py b/netbox/netbox/forms/base.py
index 43d0850f0..4720162f1 100644
--- a/netbox/netbox/forms/base.py
+++ b/netbox/netbox/forms/base.py
@@ -57,6 +57,17 @@ class NetBoxModelForm(BootstrapMixin, CheckLastUpdatedMixin, CustomFieldsMixin,
 
         return super().clean()
 
+    def _post_clean(self):
+        """
+        Override BaseModelForm's _post_clean() to store many-to-many field values on the model instance.
+        """
+        self.instance._m2m_values = {}
+        for field in self.instance._meta.local_many_to_many:
+            if field.name in self.cleaned_data:
+                self.instance._m2m_values[field.name] = list(self.cleaned_data[field.name])
+
+        return super()._post_clean()
+
 
 class NetBoxModelImportForm(CSVModelForm, NetBoxModelForm):
     """