diff --git a/tools/update-checkasm.sh b/tools/update-checkasm.sh
new file mode 100755
index 0000000000..80eda113de
--- /dev/null
+++ b/tools/update-checkasm.sh
@@ -0,0 +1,71 @@
+#!/bin/sh
+
+set -e
+
+UPDATE_COMMIT=${1:-master}
+
+cd $(dirname $0)/..
+
+trim() {
+    cd tests/checkasm/ext
+    oldifs="$IFS"
+    # Set IFS to contain only a newline, no regular space, to make the
+    # for loop below properly handle file names with spaces.
+    IFS="
+"
+    for i in $(ls -A); do
+        case "$i" in
+        include|src|LICENSE)
+            # Keep these
+            ;;
+        *)
+            if ! git rm -rf "$i"; then
+                echo "Unable to trim out $i - untracked file?"
+                exit 1
+            fi
+            ;;
+        esac
+    done
+    # Remove the meson files from the include/src subdirectories.
+    for i in $(find . -name meson.build); do
+        if ! git rm -f "$i"; then
+            echo "Unable to trim out $i - untracked file?"
+            exit 1
+        fi
+    done
+    IFS="$oldifs"
+}
+
+git update-index --refresh
+if ! git diff-index --quiet HEAD --; then
+    echo Working tree has modifications - aborting.
+    exit 1
+fi
+
+orig_commit=$(git rev-parse HEAD)
+if git subtree pull --squash --prefix=tests/checkasm/ext \
+   -m "checkasm: Update from upstream" \
+   https://code.ffmpeg.org/FFmpeg/checkasm.git ${UPDATE_COMMIT}; then
+    # Successfully merged updates (or no updates at all).
+    new_commit=$(git rev-parse HEAD)
+    trim
+    if [ "$orig_commit" = "$new_commit" ]; then
+        # Nothing merged from upstream
+        git commit -m "checkasm: Trim out unused upstream files"
+    else
+        # Merged new changes; amend the results from trimming the subtree,
+        # in case upstream added new files we don't need.
+        git commit --amend --no-edit
+    fi
+else
+    # Failed merge - possibly due to conflicts, due to updates to files that
+    # we have trimmed out. Try removing those files again, with "git rm",
+    # and check if that was enough to resolve the conflicts.
+    trim
+    if ! git commit; then
+        echo "Committing trimmed output failed - are there remaining merge conflicts?"
+        exit 1
+    else
+        echo "Resolved conflicts by re-trimming out files."
+    fi
+fi