diff --git a/setup.py b/setup.py
index d41c943d..0b96d025 100644
--- a/setup.py
+++ b/setup.py
@@ -27,7 +27,7 @@ setup(
         'Pykka >= 1.1',
         'requests >= 2.0',
         'setuptools',
-        'tornado >= 3.2',
+        'tornado >= 3.2, < 5',  # Tornado 5 requires Python >= 2.7.9
     ],
     extras_require={'http': []},
     entry_points={