diff --git a/taskcluster/docker/debian-base/Dockerfile b/taskcluster/docker/debian-base/Dockerfile
index c3eebd715cd1..c447f6ed20be 100644
--- a/taskcluster/docker/debian-base/Dockerfile
+++ b/taskcluster/docker/debian-base/Dockerfile
@@ -54,6 +54,7 @@ RUN /usr/local/sbin/setup_packages.sh $DOCKER_IMAGE_PACKAGES && \
       make \
       mercurial \
       python \
+      python3.5 \
       xz-utils
 
 # %include testing/mozharness/external_tools/robustcheckout.py