Mattermost

Switching from Slack

Your data: Slack's export (a zip) is converted with Mattermost's mmetl tool into a JSONL file plus a data/ folder of attachments, then imported with mmctl.

1. Back up your Mattermost environment, check its server settings, and decide how much history and how many attachments to bring.
2. Export your data with Slack's export tools, noting whether it's a single workspace or Enterprise Grid.
3. Run mmetl: validate the export, do transform slack --dry-run, then the full transform. Split Enterprise Grid exports with grid-transform first.
4. Package the transformed data and upload and process it with mmctl.
5. Check channels, users and history, fix unread channels and threads, and activate imported accounts before switching over.

Watch out for:

• Mattermost's guide lists this for its Entry, Professional, Enterprise and Enterprise Advanced plans.
• Slack's export options can change, and Slack may charge fees for exporting data.
• With several exports, transform each one separately and import them in date order.

Sources: docs.mattermost.com/administration-guide/onboard/migrate-from-slack.html, checked 19 Oct 2026.