diff --git a/Misc/NEWS.d/next/Library/2019-07-20-01-17-43.bpo-36161.Fzf-f9.rst b/Misc/NEWS.d/next/Library/2019-07-20-01-17-43.bpo-36161.Fzf-f9.rst
new file mode 100644
index 0000000..b31496c
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-07-20-01-17-43.bpo-36161.Fzf-f9.rst
@@ -0,0 +1 @@
+In :mod:`posix`, use ``ttyname_r`` instead of ``ttyname`` for thread safety.
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 5664027..aeb0e9d 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -2774,13 +2774,24 @@
 os_ttyname_impl(PyObject *module, int fd)
 /*[clinic end generated code: output=c424d2e9d1cd636a input=9ff5a58b08115c55]*/
 {
-    char *ret;
 
-    ret = ttyname(fd);
-    if (ret == NULL) {
+    long size = sysconf(_SC_TTY_NAME_MAX);
+    if (size == -1) {
         return posix_error();
     }
-    return PyUnicode_DecodeFSDefault(ret);
+    char *buffer = (char *)PyMem_RawMalloc(size);
+    if (buffer == NULL) {
+        return PyErr_NoMemory();
+    }
+    int ret = ttyname_r(fd, buffer, size);
+    if (ret != 0) {
+        PyMem_RawFree(buffer);
+        errno = ret;
+        return posix_error();
+    }
+    PyObject *res = PyUnicode_DecodeFSDefault(buffer);
+    PyMem_RawFree(buffer);
+    return res;
 }
 #endif