diff --git a/docs/release-notes/version-3.5.md b/docs/release-notes/version-3.5.md
index f0ad05a9a..d6e7bdb53 100644
--- a/docs/release-notes/version-3.5.md
+++ b/docs/release-notes/version-3.5.md
@@ -8,6 +8,7 @@
 * [#12384](https://github.com/netbox-community/netbox/issues/12384) - Add a three-second timeout for RSS reader widget
 * [#12395](https://github.com/netbox-community/netbox/issues/12395) - Fix "create & add another" action for objects with custom fields
 * [#12396](https://github.com/netbox-community/netbox/issues/12396) - Provider account should not be a required field in REST API serializer
+* [#12400](https://github.com/netbox-community/netbox/issues/12400) - Validate default values for object and multi-object custom fields
 * [#12401](https://github.com/netbox-community/netbox/issues/12401) - Support the creation of front ports without a pre-populated device ID
 * [#12405](https://github.com/netbox-community/netbox/issues/12405) - Fix filtering for VLAN groups displayed under site view
 * [#12412](https://github.com/netbox-community/netbox/issues/12412) - Device/VM interface MAC addresses can be nullified via REST API
diff --git a/netbox/extras/models/customfields.py b/netbox/extras/models/customfields.py
index 18430300f..439d15edc 100644
--- a/netbox/extras/models/customfields.py
+++ b/netbox/extras/models/customfields.py
@@ -606,5 +606,18 @@ class CustomField(CloningMixin, ExportTemplatesMixin, ChangeLoggedModel):
                         f"Invalid choice(s) ({', '.join(value)}). Available choices are: {', '.join(self.choices)}"
                     )
 
+            # Validate selected object
+            elif self.type == CustomFieldTypeChoices.TYPE_OBJECT:
+                if type(value) is not int:
+                    raise ValidationError(f"Value must be an object ID, not {type(value).__name__}")
+
+            # Validate selected objects
+            elif self.type == CustomFieldTypeChoices.TYPE_MULTIOBJECT:
+                if type(value) is not list:
+                    raise ValidationError(f"Value must be a list of object IDs, not {type(value).__name__}")
+                for id in value:
+                    if type(id) is not int:
+                        raise ValidationError(f"Found invalid object ID: {id}")
+
         elif self.required:
             raise ValidationError("Required field cannot be empty.")