diff --git a/Doc/library/mmap.rst b/Doc/library/mmap.rst
index 12b14d69332..1f3fbc340fc 100644
--- a/Doc/library/mmap.rst
+++ b/Doc/library/mmap.rst
@@ -244,7 +244,8 @@ To map anonymous memory, -1 should be passed as the fileno along with the length
    .. method:: readline()
 
       Returns a single line, starting at the current file position and up to the
-      next newline.
+      next newline. The file position is updated to point after the bytes that were
+      returned.
 
 
    .. method:: resize(newsize)
diff --git a/Misc/NEWS.d/next/Documentation/2020-01-17-13-59-21.bpo-39369.Bx5yE3.rst b/Misc/NEWS.d/next/Documentation/2020-01-17-13-59-21.bpo-39369.Bx5yE3.rst
new file mode 100644
index 00000000000..7f41735b9d3
--- /dev/null
+++ b/Misc/NEWS.d/next/Documentation/2020-01-17-13-59-21.bpo-39369.Bx5yE3.rst
@@ -0,0 +1 @@
+Update mmap readline method description. The fact that the readline method does update the file position should not be ignored since this might give the impression for the programmer that it doesn't update it.
\ No newline at end of file