diff --git a/Doc/library/os.rst b/Doc/library/os.rst
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -802,6 +802,7 @@
 
 Force write of file with filedescriptor *fd* to disk. On Unix, this calls the
 native :c:func:`fsync` function; on Windows, the MS :c:func:`_commit` function.
+ On Mac OS X this calls :c:func:`fcntl` with the :data:`F_FULLFSYNC` value.
 
 If you're starting with a buffered Python :term:`file object` *f*, first do
 ``f.flush()``, and then do ``os.fsync(f.fileno())``, to ensure that all internal
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -2125,7 +2125,26 @@
 static PyObject *
 posix_fsync(PyObject *self, PyObject *fdobj)
 {
+#ifdef __APPLE__
+ /* Issue 11277 revealed that OS X fsync() does not truly synchronize file
+ * data to physical backing store, whereas this fcntl does */
+ int fd;
+ int res;
+ fd = PyObject_AsFileDescriptor(fdobj);
+ if (fd < 0)
+ return NULL;
+ if (!_PyVerify_fd(fd))
+ return posix_error();
+ Py_BEGIN_ALLOW_THREADS
+ res = fcntl(fd, F_FULLFSYNC);
+ Py_END_ALLOW_THREADS
+ if (res < 0)
+ return posix_error();
+ Py_INCREF(Py_None);
+ return Py_None;
+#else
 return posix_fildes(fdobj, fsync);
+#endif
 }
 #endif /* HAVE_FSYNC */
 
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