diff -r c53d24ee3963 Lib/locale.py
--- a/Lib/locale.py	Sat Jul 25 02:45:18 2015 +0200
+++ b/Lib/locale.py	Sat Jul 25 12:43:53 2015 +0200
@@ -484,6 +484,10 @@
         return tuple(code.split('.')[:2])
     elif code == 'C':
         return None, None
+    elif code == 'UTF-8':
+        # On MacOSX "LC_CTYPE=UTF-8" is a valid locale setting
+        # for getting UTF-8 handling for text.
+        return None, 'UTF-8'
     raise ValueError('unknown locale: %s' % localename)
 
 def _build_localename(localetuple):
diff -r c53d24ee3963 Lib/test/test_locale.py
--- a/Lib/test/test_locale.py	Sat Jul 25 02:45:18 2015 +0200
+++ b/Lib/test/test_locale.py	Sat Jul 25 12:43:53 2015 +0200
@@ -477,6 +477,38 @@
 
 
 class TestMiscellaneous(unittest.TestCase):
+    def test_defaults_UTF8(self):
+        # Issue #18378: on (at least) OSX setting LC_CTYPE to "UTF-8" is
+        # valid.
+        import _locale
+        import os
+
+        self.assertEqual(locale._parse_localename('UTF-8'), (None, 'UTF-8'))
+
+        if hasattr(_locale, '_getdefaultlocale'):
+            orig_getlocale = _locale._getdefaultlocale
+            del _locale._getdefaultlocale
+        else:
+            orig_getlocale = None
+
+        orig_env = {}
+        try:
+            for key in ('LC_ALL', 'LC_CTYPE', 'LANG', 'LANGUAGE'):
+                if key in os.environ:
+                    orig_env[key] = os.environ[key]
+                    del os.environ[key]
+
+                os.environ['LC_CTYPE'] = 'UTF-8'
+
+                self.assertEqual(locale.getdefaultlocale(), (None, 'UTF-8'))
+
+        finally:
+            for k in orig_env:
+                os.environ[k] = orig_env[k]
+
+            if orig_getlocale is not None:
+                _locale._getdefaultlocale = orig_getlocale
+
     def test_getpreferredencoding(self):
         # Invoke getpreferredencoding to make sure it does not cause exceptions.
         enc = locale.getpreferredencoding()