diff --git a/doc/requirements.txt b/doc/requirements.txt
index db5793b..0ba268d 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -20,6 +20,6 @@ snowballstemmer==1.2.1
 Sphinx==1.3.4
 sphinx-autobuild==0.5.2
 sphinx-rtd-theme==0.1.9
-tornado==4.3
+tornado==6.3.3
 watchdog==0.8.3
 wheel==0.38.1