diff --git a/.github/workflows/mattermost-notify.yml b/.github/workflows/mattermost-notify.yml
new file mode 100644
index 0000000..1ac31ad
--- /dev/null
+++ b/.github/workflows/mattermost-notify.yml
@@ -0,0 +1,63 @@
+name: mattermost-notify.yml
+
+on:
+  workflow_call:
+    inputs:
+      mattermost_url:
+        type: string
+        description: "Base URL for the Mattermost instance (for example, https://example.com)"
+        default: https://chat.holochain.org
+        required: false
+      channel_id:
+        type: string
+        description: "Mattermost channel ID to post to"
+        required: true
+      message:
+        type: string
+        description: "Message text to send"
+        required: true
+    secrets:
+      MATTERMOST_PERSONAL_ACCESS_TOKEN:
+        description: "Mattermost personal access token"
+        required: true
+
+jobs:
+  notify:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Send Mattermost message
+        env:
+          MATTERMOST_URL: ${{ inputs.mattermost_url }}
+          MATTERMOST_CHANNEL_ID: ${{ inputs.channel_id }}
+          MATTERMOST_MESSAGE: ${{ inputs.message }}
+          MATTERMOST_PERSONAL_ACCESS_TOKEN: ${{ secrets.MATTERMOST_PERSONAL_ACCESS_TOKEN }}
+        run: |
+          set -euo pipefail
+
+          base_url="${MATTERMOST_URL%/}"
+          posts_url="${base_url}/api/v4/posts"
+
+          payload="$(jq -cn \
+            --arg channel_id "$MATTERMOST_CHANNEL_ID" \
+            --arg message "$MATTERMOST_MESSAGE" \
+            '{channel_id: $channel_id, message: $message}')"
+
+          response_body="$(mktemp)"
+
+          response_code="$(curl \
+            --silent \
+            --show-error \
+            --output "$response_body" \
+            --write-out "%{http_code}" \
+            --request POST "$posts_url" \
+            --header "Authorization: Bearer $MATTERMOST_PERSONAL_ACCESS_TOKEN" \
+            --header "Content-Type: application/json" \
+            --data "$payload")"
+
+          if [ "$response_code" -lt 200 ] || [ "$response_code" -ge 300 ]; then
+            printf 'Mattermost API request failed with HTTP %s\n' "$response_code"
+            cat "$response_body"
+            exit 1
+          fi
+
+          printf 'Message sent to Mattermost channel %s\n' "$MATTERMOST_CHANNEL_ID"
diff --git a/README.md b/README.md
index 99ff228..eb004f7 100644
--- a/README.md
+++ b/README.md
@@ -1,2 +1,33 @@
 # actions
 Actions for common tasks in Holochain repositories
+
+## Mattermost notifier action
+
+Reusable workflow: `.github/workflows/mattermost-notify.yml`
+
+Inputs:
+- `mattermost_url`: base URL for Mattermost (defaults to `https://chat.holochain.org`)
+- `channel_id`: target Mattermost channel ID. This can be obtained by clicking the "(i)" icon at the top right of an open channel.
+- `message`: message text to post
+
+Secret:
+- `MATTERMOST_PERSONAL_ACCESS_TOKEN`: Mattermost personal access token with permission to post in the channel.
+
+Example usage:
+
+```yaml
+name: Notify Mattermost
+
+on:
+  workflow_dispatch:
+
+jobs:
+  notify:
+    uses: holochain/actions/.github/workflows/mattermost-notify.yml@main
+    with:
+      mattermost_url: https://chat.example.com
+      channel_id: your_channel_id
+      message: Hello from GitHub Actions
+    secrets:
+      MATTERMOST_PERSONAL_ACCESS_TOKEN: ${{ secrets.MATTERMOST_PERSONAL_ACCESS_TOKEN }}
+```