diff --git a/optional_requirements.txt b/optional_requirements.txt
index fe6503c5..8c491b58 100644
--- a/optional_requirements.txt
+++ b/optional_requirements.txt
@@ -3,3 +3,4 @@ imread; python_version != '2.7' and python_version != '3.2'
 SimpleITK; python_version != '3.2' and python_version != '3.4'
 astropy
 tifffile
+imageio