diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index 3d031534e93..925a722f208 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -1804,6 +1804,8 @@ features:
    This function can also support :ref:`paths relative to directory descriptors
    <dir_fd>`.
 
+   Availability: Unix.
+
    .. versionadded:: 3.3
       The *dir_fd* argument.