diff --git a/.github/workflows/release-appimage.yml b/.github/workflows/release-appimage.yml
index 11c13d5..19aa06d 100644
--- a/.github/workflows/release-appimage.yml
+++ b/.github/workflows/release-appimage.yml
@@ -44,7 +44,9 @@ jobs:
       - uses: actions/upload-artifact@cf430e030ddbb5b0abf93d22962f4752f3646cd9 # v7
         with:
           name: appimage-x86_64
-          path: dist/*.AppImage
+          path: |
+            dist/*.AppImage
+            dist/*.AppImage.zsync
           if-no-files-found: error
 
   # Fedora container because Arch has no official ARM64 image and Arch Linux ARM lacks
@@ -77,5 +79,7 @@ jobs:
       - uses: actions/upload-artifact@cf430e030ddbb5b0abf93d22962f4752f3646cd9 # v7
         with:
           name: appimage-aarch64
-          path: dist/*.AppImage
+          path: |
+            dist/*.AppImage
+            dist/*.AppImage.zsync
           if-no-files-found: error
diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index be020bf..e83277b 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -173,16 +173,18 @@ jobs:
           for arch in x86_64 aarch64; do
             test -s "dist/zaptide-${GITHUB_REF_NAME}-${arch}.flatpak"
             test -s "dist/zaptide-${GITHUB_REF_NAME}-${arch}.AppImage"
+            test -s "dist/zaptide-${GITHUB_REF_NAME}-${arch}.AppImage.zsync"
             test -s "dist/zaptide-${GITHUB_REF_NAME}-${arch}-linux-gnu.tar.gz"
           done
       - name: Checksums
         working-directory: dist
-        run: sha256sum *.flatpak *.AppImage *.tar.gz > checksums.txt
+        run: sha256sum *.flatpak *.AppImage *.AppImage.zsync *.tar.gz > checksums.txt
       - uses: softprops/action-gh-release@efb35369e0ad2afab669f228072c1b0d510eae64 # v3.0.3
         with:
           files: |
             dist/*.flatpak
             dist/*.AppImage
+            dist/*.AppImage.zsync
             dist/*.tar.gz
             dist/checksums.txt
           fail_on_unmatched_files: true
diff --git a/PACKAGING.md b/PACKAGING.md
index 43ac356..88ee7ae 100644
--- a/PACKAGING.md
+++ b/PACKAGING.md
@@ -68,6 +68,16 @@ and Arch Linux ARM ships libadwaita 1.9. `build.sh` picks the `dnf` path when
 podman run --rm -v "$PWD":/src -w /src -e VERSION=dev registry.fedoraproject.org/fedora:45 ./packaging/appimage/build.sh
 ```
 
+Each AppImage embeds the update information
+`gh-releases-zsync|luminusOS|zaptide|latest|zaptide-*-<arch>.AppImage.zsync`, and
+the build writes the matching `.zsync` file next to it. The release publishes both,
+so [AppImageUpdate](https://github.com/AppImageCommunity/AppImageUpdate) and
+`appimageupdatetool` download only the changed blocks:
+
+```sh
+appimageupdatetool zaptide.AppImage
+```
+
 ## Build dependencies
 
 | Dependency | Fedora | Notes |
diff --git a/README.md b/README.md
index ee05f51..91f9825 100644
--- a/README.md
+++ b/README.md
@@ -80,6 +80,10 @@ chmod +x ./zaptide.AppImage
 ./zaptide.AppImage
 ```
 
+Update an AppImage in place with
+[AppImageUpdate](https://github.com/AppImageCommunity/AppImageUpdate); it
+downloads only what changed.
+
 On first launch, use WhatsApp's **Linked devices** menu to scan the QR code or
 link with your phone number. Recent history may take a few minutes to arrive.
 
diff --git a/packaging/appimage/build.sh b/packaging/appimage/build.sh
index 46f875f..0f02963 100755
--- a/packaging/appimage/build.sh
+++ b/packaging/appimage/build.sh
@@ -2,7 +2,7 @@
 # Builds the ZapTide AppImage with quick-sharun. Run it as root inside an Arch
 # Linux container (Arch ships libadwaita 1.10 and has no 32-bit libs in /usr/lib):
 #   podman run --rm -v "$PWD":/src -w /src archlinux ./packaging/appimage/build.sh
-# The result is dist/zaptide-$ARCH.AppImage.
+# The result is dist/zaptide-$VERSION-$ARCH.AppImage and its .zsync file.
 set -eux
 
 ARCH="$(uname -m)"
@@ -42,6 +42,10 @@ export ICON=data/icons/zaptide.svg
 export DESKTOP=data/dev.luminusos.ZapTide.desktop
 export STARTUPWMCLASS=dev.luminusos.ZapTide
 export GTK_CLASS_FIX=1
+# Delta updates: AppImageUpdate reads this from the AppImage and fetches the
+# .zsync published next to it in the latest GitHub release.
+export UPINFO="gh-releases-zsync|luminusOS|zaptide|latest|zaptide-*-$ARCH.AppImage.zsync"
 
 ./quick-sharun /usr/bin/zaptide
 ./quick-sharun --make-appimage
+test -s "$OUTPATH/$OUTNAME.zsync"