diff --git a/Lib/test/test_utf8_mode.py b/Lib/test/test_utf8_mode.py
index 26e2e13ec53..4a16b730468 100644
--- a/Lib/test/test_utf8_mode.py
+++ b/Lib/test/test_utf8_mode.py
@@ -12,6 +12,7 @@
 
 
 MS_WINDOWS = (sys.platform == 'win32')
+POSIX_LOCALES = ('C', 'POSIX')
 
 
 class UTF8ModeTests(unittest.TestCase):
@@ -23,7 +24,7 @@ class UTF8ModeTests(unittest.TestCase):
 
     def posix_locale(self):
         loc = locale.setlocale(locale.LC_CTYPE, None)
-        return (loc == 'C')
+        return (loc in POSIX_LOCALES)
 
     def get_output(self, *args, failure=False, **kw):
         kw = dict(self.DEFAULT_ENV, **kw)
@@ -39,8 +40,10 @@ def get_output(self, *args, failure=False, **kw):
     def test_posix_locale(self):
         code = 'import sys; print(sys.flags.utf8_mode)'
 
-        out = self.get_output('-c', code, LC_ALL='C')
-        self.assertEqual(out, '1')
+        for loc in POSIX_LOCALES:
+            with self.subTest(LC_ALL=loc):
+                out = self.get_output('-c', code, LC_ALL=loc)
+                self.assertEqual(out, '1')
 
     def test_xoption(self):
         code = 'import sys; print(sys.flags.utf8_mode)'
@@ -201,8 +204,10 @@ def test_locale_getpreferredencoding(self):
         out = self.get_output('-X', 'utf8', '-c', code)
         self.assertEqual(out, 'UTF-8 UTF-8')
 
-        out = self.get_output('-X', 'utf8', '-c', code, LC_ALL='C')
-        self.assertEqual(out, 'UTF-8 UTF-8')
+        for loc in POSIX_LOCALES:
+            with self.subTest(LC_ALL=loc):
+                out = self.get_output('-X', 'utf8', '-c', code, LC_ALL=loc)
+                self.assertEqual(out, 'UTF-8 UTF-8')
 
     @unittest.skipIf(MS_WINDOWS, 'test specific to Unix')
     def test_cmd_line(self):
@@ -217,11 +222,17 @@ def check(utf8_opt, expected, **kw):
             self.assertEqual(args, ascii(expected), out)
 
         check('utf8', [arg_utf8])
+        for loc in POSIX_LOCALES:
+            with self.subTest(LC_ALL=loc):
+                check('utf8', [arg_utf8], LC_ALL=loc)
+
         if sys.platform == 'darwin' or support.is_android:
             c_arg = arg_utf8
         else:
             c_arg = arg_ascii
-        check('utf8=0', [c_arg], LC_ALL='C')
+        for loc in POSIX_LOCALES:
+            with self.subTest(LC_ALL=loc):
+                check('utf8=0', [c_arg], LC_ALL=loc)
 
     def test_optim_level(self):
         # CPython: check that Py_Main() doesn't increment Py_OptimizeFlag
diff --git a/Misc/NEWS.d/next/Core and Builtins/2018-08-28-11-52-13.bpo-34527.sh5MQJ.rst b/Misc/NEWS.d/next/Core and Builtins/2018-08-28-11-52-13.bpo-34527.sh5MQJ.rst
new file mode 100644
index 00000000000..280a8922edc
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2018-08-28-11-52-13.bpo-34527.sh5MQJ.rst	
@@ -0,0 +1,2 @@
+The UTF-8 Mode is now also enabled by the "POSIX" locale, not only by the "C"
+locale.
diff --git a/Misc/NEWS.d/next/Core and Builtins/2018-08-28-11-53-39.bpo-34527.aBEX9b.rst b/Misc/NEWS.d/next/Core and Builtins/2018-08-28-11-53-39.bpo-34527.aBEX9b.rst
new file mode 100644
index 00000000000..9fce794305c
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2018-08-28-11-53-39.bpo-34527.aBEX9b.rst	
@@ -0,0 +1,3 @@
+On FreeBSD, Py_DecodeLocale() and Py_EncodeLocale() now also forces the
+ASCII encoding if the LC_CTYPE locale is "POSIX", not only if the LC_CTYPE
+locale is "C".
diff --git a/Modules/main.c b/Modules/main.c
index 45148dc9839..856f92c9d9b 100644
--- a/Modules/main.c
+++ b/Modules/main.c
@@ -2102,15 +2102,25 @@ pymain_read_conf(_PyMain *pymain, _Py_CommandLineDetails *cmdline)
 static void
 config_init_locale(_PyCoreConfig *config)
 {
-    if (_Py_LegacyLocaleDetected()) {
-        /* POSIX locale: enable C locale coercion and UTF-8 Mode */
-        if (config->utf8_mode < 0) {
-            config->utf8_mode = 1;
-        }
-        if (config->coerce_c_locale < 0) {
+    if (config->coerce_c_locale < 0) {
+        /* The C locale enables the C locale coercion (PEP 538) */
+        if (_Py_LegacyLocaleDetected()) {
             config->coerce_c_locale = 1;
         }
     }
+
+#ifndef MS_WINDOWS
+    if (config->utf8_mode < 0) {
+        /* The C locale and the POSIX locale enable the UTF-8 Mode (PEP 540) */
+        const char *ctype_loc = setlocale(LC_CTYPE, NULL);
+        if (ctype_loc != NULL
+           && (strcmp(ctype_loc, "C") == 0
+               || strcmp(ctype_loc, "POSIX") == 0))
+        {
+            config->utf8_mode = 1;
+        }
+    }
+#endif
 }
 
 
diff --git a/Python/fileutils.c b/Python/fileutils.c
index 35869c81ac9..b413f4e1e68 100644
--- a/Python/fileutils.c
+++ b/Python/fileutils.c
@@ -128,7 +128,7 @@ check_force_ascii(void)
     loc = setlocale(LC_CTYPE, NULL);
     if (loc == NULL)
         goto error;
-    if (strcmp(loc, "C") != 0) {
+    if (strcmp(loc, "C") != 0 && strcmp(loc, "POSIX") != 0) {
         /* the LC_CTYPE locale is different than C */
         return 0;
     }