diff --git a/falcon/util/uri.py b/falcon/util/uri.py
index f014e5a..2359672 100644
--- a/falcon/util/uri.py
+++ b/falcon/util/uri.py
@@ -178,9 +178,12 @@ if six.PY2:
         tokens = decoded_uri.split('%')
         decoded_uri = tokens[0]
         for token in tokens[1:]:
-            char, byte = _HEX_TO_BYTE[token[:2]]
-            decoded_uri += char + token[2:]
-
+            token_partial = token[:2]
+            if token_partial in _HEX_TO_BYTE:
+                char, byte = _HEX_TO_BYTE[token_partial]
+            else:
+                char, byte = '%', 0
+            decoded_uri += char + (token[2:] if byte else token)
             only_ascii = only_ascii and (byte <= 127)
 
         # PERF(kgriffs): Only spend the time to do this if there
@@ -235,7 +238,12 @@ else:
         tokens = decoded_uri.split(b'%')
         decoded_uri = tokens[0]
         for token in tokens[1:]:
-            decoded_uri += _HEX_TO_BYTE[token[:2]] + token[2:]
+            token_partial = token[:2]
+            if token_partial in _HEX_TO_BYTE:
+                decoded_uri += _HEX_TO_BYTE[token_partial] + token[2:]
+            else:
+                # malformed percentage like "x=%" or "y=%+"
+                decoded_uri += b'%' + token
 
         # Convert back to str
         return decoded_uri.decode('utf-8', 'replace')
diff --git a/tests/test_query_params.py b/tests/test_query_params.py
index 5604e9a..50ed010 100644
--- a/tests/test_query_params.py
+++ b/tests/test_query_params.py
@@ -64,6 +64,16 @@ class _TestQueryParams(testing.TestBase):
         self.assertEqual(req.get_param_as_list('id', int), [23, 42])
         self.assertEqual(req.get_param('q'), u'\u8c46 \u74e3')
 
+    def test_bad_percentage(self):
+        query_string = 'x=%%20%+%&y=peregrine&z=%a%z%zz%1%20e'
+        self.simulate_request('/', query_string=query_string)
+        self.assertEqual(self.srmock.status, falcon.HTTP_200)
+
+        req = self.resource.req
+        self.assertEqual(req.get_param('x'), '% % %')
+        self.assertEqual(req.get_param('y'), 'peregrine')
+        self.assertEqual(req.get_param('z'), '%a%z%zz%1 e')
+
     def test_allowed_names(self):
         query_string = ('p=0&p1=23&2p=foo&some-thing=that&blank=&'
                         'some_thing=x&-bogus=foo&more.things=blah&'