1 files changed, 7 insertions, 0 deletions
diff --git a/sphinx/util/jsonimpl.py b/sphinx/util/jsonimpl.py
index ecb0b791a..4fb8e1f5d 100644
--- a/sphinx/util/jsonimpl.py
+++ b/sphinx/util/jsonimpl.py
@@ -9,13 +9,20 @@
 """
 
 import json
+import warnings
 from collections import UserString
 
+from sphinx.deprecation import RemovedInSphinx40Warning
+
 if False:
     # For type annotation
     from typing import Any, IO  # NOQA
 
 
+warnings.warn('sphinx.util.jsonimpl is deprecated',
+              RemovedInSphinx40Warning, stacklevel=2)
+
+
 class SphinxJSONEncoder(json.JSONEncoder):
     """JSONEncoder subclass that forces translation proxies."""
     def default(self, obj):