diff --git a/Misc/NEWS.d/next/Documentation/2020-06-17-14-47-48.bpo-25377.CTxC6o.rst b/Misc/NEWS.d/next/Documentation/2020-06-17-14-47-48.bpo-25377.CTxC6o.rst
new file mode 100644
index 00000000000..019a1c42d88
--- /dev/null
+++ b/Misc/NEWS.d/next/Documentation/2020-06-17-14-47-48.bpo-25377.CTxC6o.rst
@@ -0,0 +1 @@
+Clarify use of octal format of mode argument in help(os.chmod) as well as help(os.fchmod)
diff --git a/Modules/clinic/posixmodule.c.h b/Modules/clinic/posixmodule.c.h
index 86251008b1b..d4722cc533c 100644
--- a/Modules/clinic/posixmodule.c.h
+++ b/Modules/clinic/posixmodule.c.h
@@ -501,6 +501,9 @@ PyDoc_STRVAR(os_chmod__doc__,
 "    If this functionality is unavailable, using it raises an exception.\n"
 "  mode\n"
 "    Operating-system mode bitfield.\n"
+"    Be careful when using number literals for *mode*. The conventional UNIX notation for\n"
+"    numeric modes uses an octal base, which needs to be indicated with a ``0o`` prefix in\n"
+"    Python.\n"
 "  dir_fd\n"
 "    If not None, it should be a file descriptor open to a directory,\n"
 "    and path should be relative; path will then be relative to that\n"
@@ -602,6 +605,14 @@ PyDoc_STRVAR(os_fchmod__doc__,
 "\n"
 "Change the access permissions of the file given by file descriptor fd.\n"
 "\n"
+"  fd\n"
+"    The file descriptor of the file to be modified.\n"
+"  mode\n"
+"    Operating-system mode bitfield.\n"
+"    Be careful when using number literals for *mode*. The conventional UNIX notation for\n"
+"    numeric modes uses an octal base, which needs to be indicated with a ``0o`` prefix in\n"
+"    Python.\n"
+"\n"
 "Equivalent to os.chmod(fd, mode).");
 
 #define OS_FCHMOD_METHODDEF    \
@@ -11549,4 +11560,4 @@ exit:
 #ifndef OS_WAITSTATUS_TO_EXITCODE_METHODDEF
     #define OS_WAITSTATUS_TO_EXITCODE_METHODDEF
 #endif /* !defined(OS_WAITSTATUS_TO_EXITCODE_METHODDEF) */
-/*[clinic end generated code: output=04fd23c89ab41f75 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=41eab6c3523792a9 input=a9049054013a1b77]*/
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 4817973262f..607d40b59d9 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -3173,6 +3173,9 @@ os.chmod
 
     mode: int
         Operating-system mode bitfield.
+        Be careful when using number literals for *mode*. The conventional UNIX notation for
+        numeric modes uses an octal base, which needs to be indicated with a ``0o`` prefix in
+        Python.
 
     *
 
@@ -3198,7 +3201,7 @@ dir_fd and follow_symlinks may not be implemented on your platform.
 static PyObject *
 os_chmod_impl(PyObject *module, path_t *path, int mode, int dir_fd,
               int follow_symlinks)
-/*[clinic end generated code: output=5cf6a94915cc7bff input=989081551c00293b]*/
+/*[clinic end generated code: output=5cf6a94915cc7bff input=674a14bc998de09d]*/
 {
     int result;
 
@@ -3328,7 +3331,12 @@ os_chmod_impl(PyObject *module, path_t *path, int mode, int dir_fd,
 os.fchmod
 
     fd: int
+        The file descriptor of the file to be modified.
     mode: int
+        Operating-system mode bitfield.
+        Be careful when using number literals for *mode*. The conventional UNIX notation for
+        numeric modes uses an octal base, which needs to be indicated with a ``0o`` prefix in
+        Python.
 
 Change the access permissions of the file given by file descriptor fd.
 
@@ -3337,7 +3345,7 @@ Equivalent to os.chmod(fd, mode).
 
 static PyObject *
 os_fchmod_impl(PyObject *module, int fd, int mode)
-/*[clinic end generated code: output=afd9bc05b4e426b3 input=8ab11975ca01ee5b]*/
+/*[clinic end generated code: output=afd9bc05b4e426b3 input=b5594618bbbc22df]*/
 {
     int res;
     int async_err = 0;