diff --git a/.travis.yml b/.travis.yml
index 15ad1e4..6e929e6 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -7,11 +7,7 @@ language: c
 compiler:
   - gcc
   - clang
-addons:
-  apt:
-    packages:
-    - clang-format-3.5
 script:
-  - clang-format-3.5 -i *.[ch] && git diff --exit-code || (echo 'Code was not formatted using clang-format!'; false)
   - docker build --pull --no-cache --rm -t=i3lock -f travis/Dockerfile .
+  - docker run -e CC=$CC -v $PWD:/usr/src:rw i3lock /bin/sh -c 'clang-format-9 -i *.[ch] && git diff --exit-code || (echo "Code was not formatted using clang-format!"; false)'
   - docker run -e CC=$CC -v $PWD:/usr/src:rw i3lock /bin/sh -c 'autoreconf -fi && mkdir -p build && cd build && (../configure || (cat config.log; false)) && make -j V=1 CFLAGS="-Wformat -Wformat-security -Wextra -Wno-unused-parameter -Werror"'
diff --git a/travis/Dockerfile b/travis/Dockerfile
index 5381276..cc2dd7d 100644
--- a/travis/Dockerfile
+++ b/travis/Dockerfile
@@ -11,7 +11,7 @@ RUN echo 'APT::Acquire::Retries "5";' > /etc/apt/apt.conf.d/80retry
 # (3608 kB/s)). Hence, let’s stick with httpredir.debian.org (default) for now.
 
 # Install mk-build-deps (for installing the i3 build dependencies),
-# clang and clang-format-3.8 (for checking formatting and building with clang),
+# clang and clang-format-9 (for checking formatting and building with clang),
 # lintian (for checking spelling errors),
 # test suite dependencies (for running tests)
 RUN apt-get update && \
@@ -19,7 +19,7 @@ RUN apt-get update && \
     build-essential clang git autoconf automake libxcb-randr0-dev pkg-config libpam0g-dev \
     libcairo2-dev libxcb1-dev libxcb-dpms0-dev libxcb-image0-dev libxcb-util0-dev \
     libxcb-xrm-dev libev-dev libxcb-xinerama0-dev libxcb-xkb-dev libxkbcommon-dev \
-    libxkbcommon-x11-dev && \
+    libxkbcommon-x11-dev clang-format-9 && \
     rm -rf /var/lib/apt/lists/*
 
 WORKDIR /usr/src