diff --git a/Lib/packaging/config.py b/Lib/packaging/config.py
--- a/Lib/packaging/config.py
+++ b/Lib/packaging/config.py
@@ -282,7 +282,7 @@ class Config:
 
         for filename in filenames:
             logger.debug("  reading %s", filename)
-            parser.read(filename)
+            parser.read(filename, encoding='utf-8')
 
             if os.path.split(filename)[-1] == 'setup.cfg':
                 self._read_setup_cfg(parser, filename)
diff --git a/Lib/packaging/create.py b/Lib/packaging/create.py
--- a/Lib/packaging/create.py
+++ b/Lib/packaging/create.py
@@ -276,7 +276,7 @@ class MainProgram:
                 return
             shutil.move(_FILENAME, '%s.old' % _FILENAME)
 
-        with open(_FILENAME, 'w') as fp:
+        with open(_FILENAME, 'w', encoding='utf-8') as fp:
             fp.write('[metadata]\n')
             # simple string entries
             for name in ('name', 'version', 'summary', 'download_url'):
diff --git a/Lib/packaging/metadata.py b/Lib/packaging/metadata.py
--- a/Lib/packaging/metadata.py
+++ b/Lib/packaging/metadata.py
@@ -3,6 +3,7 @@
 Supports all metadata formats (1.0, 1.1, 1.2).
 """
 
+import codecs
 import re
 import logging
 
@@ -330,11 +331,16 @@ class Metadata:
 
     def write(self, filepath):
         """Write the metadata fields to filepath."""
-        with open(filepath, 'w') as fp:
+        with open(filepath, 'w', encoding='utf-8') as fp:
             self.write_file(fp)
 
     def write_file(self, fileobject):
         """Write the PKG-INFO format data to a file object."""
+        if not isinstance(fileobject, StringIO):
+            encoding = codecs.lookup(fileobject.encoding).name
+            if encoding != 'utf-8':
+                raise ValueError("Output file should be a UTF-8 text file, not %s encoding" % encoding)
+
         self._set_best_version()
         for field in _version2fieldlist(self['Metadata-Version']):
             values = self.get(field)
diff --git a/Lib/packaging/tests/support.py b/Lib/packaging/tests/support.py
--- a/Lib/packaging/tests/support.py
+++ b/Lib/packaging/tests/support.py
@@ -146,7 +146,7 @@ class TempdirManager:
         """
         if isinstance(path, (list, tuple)):
             path = os.path.join(*path)
-        f = open(path, 'w')
+        f = open(path, 'w', encoding='utf8')
         try:
             f.write(content)
         finally:
diff --git a/Lib/packaging/tests/test_create.py b/Lib/packaging/tests/test_create.py
--- a/Lib/packaging/tests/test_create.py
+++ b/Lib/packaging/tests/test_create.py
@@ -129,7 +129,7 @@ class CreateTestCase(support.TempdirMana
         sys.stdin.seek(0)
         main()
 
-        with open(os.path.join(self.wdir, 'setup.cfg')) as fp:
+        with open(os.path.join(self.wdir, 'setup.cfg'), encoding='utf-8') as fp:
             lines = set(line.rstrip() for line in fp)
 
         # FIXME don't use sets
@@ -205,7 +205,7 @@ ho, baby!
         sys.stdin.seek(0)
         # FIXME Out of memory error.
         main()
-        with open(os.path.join(self.wdir, 'setup.cfg')) as fp:
+        with open(os.path.join(self.wdir, 'setup.cfg'), encoding='utf-8') as fp:
             lines = set(line.rstrip() for line in fp)
 
         self.assertEqual(lines, set(['',
diff --git a/Lib/packaging/tests/test_dist.py b/Lib/packaging/tests/test_dist.py
--- a/Lib/packaging/tests/test_dist.py
+++ b/Lib/packaging/tests/test_dist.py
@@ -78,7 +78,7 @@ class DistributionTestCase(support.Tempd
         # let's make sure the file can be written
         # with Unicode fields. they are encoded with
         # PKG_INFO_ENCODING
-        with open(my_file, 'w') as fp:
+        with open(my_file, 'w', encoding='utf-8') as fp:
             dist.metadata.write_file(fp)
 
         # regular ascii is of course always usable
@@ -88,7 +88,7 @@ class DistributionTestCase(support.Tempd
                           'summary': 'Cafe torrefie',
                           'description': 'Hehehe'})
 
-        with open(my_file, 'w') as fp:
+        with open(my_file, 'w', encoding='utf-8') as fp:
             dist.metadata.write_file(fp)
 
     def test_bad_attr(self):