diff --git a/Lib/optparse.py b/Lib/optparse.py
index bec492d8cdbf1f2..733cdc230f9ed96 100644
--- a/Lib/optparse.py
+++ b/Lib/optparse.py
@@ -285,7 +285,7 @@ def expand_default(self, option):
         if default_value is NO_DEFAULT or default_value is None:
             default_value = self.NO_DEFAULT_VALUE
 
-        return option.help.replace(self.default_tag, str(default_value))
+        return option.help.replace(self.default_tag, unicode(default_value))
 
     def format_option(self, option):
         # The help for each option consists of two parts:
diff --git a/Lib/test/test_optparse.py b/Lib/test/test_optparse.py
index dc2ef0b78675169..6551e2839e75f17 100644
--- a/Lib/test/test_optparse.py
+++ b/Lib/test/test_optparse.py
@@ -613,6 +613,15 @@ def test_float_default(self):
             "  -p PROB, --prob=PROB  blow up with probability PROB [default: 0.43]\n"
         self.assertHelp(self.parser, expected_help)
 
+    def test_unicode_default(self):
+        self.parser.add_option(
+            "-p", "--prob",
+            help="blow up with probability PROB [default: %default]")
+        self.parser.set_defaults(prob=u"ol\u00E9!")
+        expected_help = self.help_prefix + \
+            u"  -p PROB, --prob=PROB  blow up with probability PROB [default: ol\u00E9!]\n"
+        self.assertHelp(self.parser, expected_help)
+
     def test_alt_expand(self):
         self.parser.add_option("-f", "--file",
                                default="foo.txt",
diff --git a/Misc/NEWS.d/next/Library/2018-09-23-19-44-54.bpo-24307.EAcncD.rst b/Misc/NEWS.d/next/Library/2018-09-23-19-44-54.bpo-24307.EAcncD.rst
new file mode 100644
index 000000000000000..32e3d3c23c633ac
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2018-09-23-19-44-54.bpo-24307.EAcncD.rst
@@ -0,0 +1,2 @@
+Encode default value before replacing it in the option. Based on patch by
+tanbro-liu.