diff --git a/awscli/shorthand.py b/awscli/shorthand.py
index 8d71354e771a..374b12d08f03 100644
--- a/awscli/shorthand.py
+++ b/awscli/shorthand.py
@@ -153,8 +153,8 @@ def _keyval(self):
         return {key: values}
 
     def _key(self):
-        # key = 1*(alpha / %x30-39)  ; [a-zA-Z0-9\-]
-        valid_chars = string.ascii_letters + string.digits + '-'
+        # key = 1*(alpha / %x30-39 / %x5f)  ; [a-zA-Z0-9\-_]
+        valid_chars = string.ascii_letters + string.digits + '-_'
         start = self._index
         while not self._at_eof():
             if self._current() not in valid_chars:
diff --git a/tests/unit/test_shorthand.py b/tests/unit/test_shorthand.py
index 4aac48960eb4..72437eb9dc62 100644
--- a/tests/unit/test_shorthand.py
+++ b/tests/unit/test_shorthand.py
@@ -50,6 +50,9 @@ def test_parse():
     # Dashes are allowed in key names.
     yield (_can_parse, 'with-dash=bar', {'with-dash': 'bar'})
 
+    # Underscore are also allowed.
+    yield (_can_parse, 'with_underscore=bar', {'with_underscore': 'bar'})
+
     # Explicit lists.
     yield (_can_parse, 'foo=[]', {'foo': []})
     yield (_can_parse, 'foo=[a]', {'foo': ['a']})