diff --git a/optional_requirements.txt b/optional_requirements.txt
index 8bc53bb0..f2611edb 100644
--- a/optional_requirements.txt
+++ b/optional_requirements.txt
@@ -4,3 +4,4 @@ SimpleITK; python_version != '3.4' and python_version != '3.5'
 astropy
 tifffile
 imageio
+PyWavelet