diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 3d1a39a86c784..75e57c2a4447f 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -2150,8 +2150,12 @@ def to_parquet(
             Name of the compression to use. Use ``None`` for no compression.
         index : bool, default None
             If ``True``, include the dataframe's index(es) in the file output.
-            If ``False``, they will not be written to the file. If ``None``,
-            the behavior depends on the chosen engine.
+            If ``False``, they will not be written to the file.
+            If ``None``, similar to ``True`` the dataframe's index(es)
+            will be saved. However, instead of being saved as values,
+            the RangeIndex will be stored as a range in the metadata so it
+            doesn't require much space and is faster. Other indexes will
+            be included as columns in the file output.
 
             .. versionadded:: 0.24.0
 
diff --git a/pandas/io/parquet.py b/pandas/io/parquet.py
index 6fc70e9f4a737..407305da30d11 100644
--- a/pandas/io/parquet.py
+++ b/pandas/io/parquet.py
@@ -230,8 +230,12 @@ def to_parquet(
         Name of the compression to use. Use ``None`` for no compression.
     index : bool, default None
         If ``True``, include the dataframe's index(es) in the file output. If
-        ``False``, they will not be written to the file. If ``None``, the
-        engine's default behavior will be used.
+        ``False``, they will not be written to the file.
+        If ``None``, similar to ``True`` the dataframe's index(es)
+        will be saved. However, instead of being saved as values,
+        the RangeIndex will be stored as a range in the metadata so it
+        doesn't require much space and is faster. Other indexes will
+        be included as columns in the file output.
 
         .. versionadded:: 0.24.0