diff --git a/netbox/extras/models/customfields.py b/netbox/extras/models/customfields.py
index 439d15edc..635f9c882 100644
--- a/netbox/extras/models/customfields.py
+++ b/netbox/extras/models/customfields.py
@@ -81,8 +81,8 @@ class CustomField(CloningMixin, ExportTemplatesMixin, ChangeLoggedModel):
         help_text=_('Internal field name'),
         validators=(
             RegexValidator(
-                regex=r'^[a-z0-9_]+$',
-                message="Only alphanumeric characters and underscores are allowed.",
+                regex=r'^(?!.*__.*)^[a-z0-9_]+$',
+                message="Only alphanumeric characters and underscores (no double-underscores) are allowed.",
                 flags=re.IGNORECASE
             ),
         )