diff --git a/requirements.txt b/requirements.txt
index c99e11623..5592626d6 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -4,3 +4,4 @@ brotli==1.0.1; platform_python_implementation != "PyPy"
 brotlipy==0.7.0; platform_python_implementation == "PyPy"
 unicodedata2==10.0.0; python_version < '3.7' and platform_python_implementation != "PyPy"
 munkres==1.0.10
+zopfli==0.1.4