diff --git a/setup.cfg b/setup.cfg
index 1e58d4581..2367c0162 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -32,7 +32,7 @@ include_package_data = True
 zip_safe = False
 packages = sphinx_rtd_theme
 install_requires =
-  sphinx >=1.6
+  sphinx >=1.6,<6
   docutils <0.18
   Jinja2 <3.1
 tests_require =