diff --git a/Doc/library/base64.rst b/Doc/library/base64.rst
index 06f3ab1282f..c08df15303e 100644
--- a/Doc/library/base64.rst
+++ b/Doc/library/base64.rst
@@ -140,6 +140,8 @@ The legacy interface:
    encoded data, and return a byte string containing the resulting binary data.
    ``decodestring`` is a deprecated alias.
 
+   .. versionadded:: 3.1
+
 
 .. function:: encode(input, output)