diff --git a/pre_commit/clientlib/validate_config.py b/pre_commit/clientlib/validate_config.py
index e4a90a6c..e4cbbddf 100644
--- a/pre_commit/clientlib/validate_config.py
+++ b/pre_commit/clientlib/validate_config.py
@@ -39,6 +39,10 @@ CONFIG_JSON_SCHEMA = {
                             'type': 'array',
                             'items': {'type': 'string'},
                         },
+                        'additional_dependencies': {
+                            'type': 'array',
+                            'items': {'type': 'string'}
+                        }
                     },
                     'required': ['id'],
                 }