Mattermost
FreeSlack on your own server, with a free edition that keeps your full history
A self-hosted team chat platform with channels, direct messages, threaded replies, files, search and webhook integrations, working much like Slack. The server is a Linux binary plus a PostgreSQL database, with desktop, web and mobile clients. The free option is the open-source Team Edition, capped at 250 active users and without single sign-on.
Why we picked it
Server source is AGPL-3.0 and the official Team Edition build is released under MIT; 39,000 stars. Version 11.11.1 shipped in September 2026, and releases are frequent.
Its most concrete use is moving team chat back onto your own machine. Messages, files and the member list sit in your database, so problems like Slack's free plan hiding everything older than 90 days do not exist here. Slack users have almost nothing to learn: channels, @mentions, threaded replies, reactions and Markdown are all there, and incoming/outgoing webhooks and slash commands connect CI, monitoring alerts and other internal systems.
For deployment, the server is a single Linux binary backed by PostgreSQL, and there is an official
Docker image, mattermost/mattermost-team-edition. Members connect to your server address with the
official desktop app (Windows, macOS, Linux), the mobile apps or a browser.
Read the free-versus-paid line carefully; this is where people get caught:
- The download page on the main site installs Mattermost Entry, not Team Edition. Entry is a
free edition under a commercial licence that shows only the most recent 10,000 channel messages
across the whole server; older messages stay in the database but cannot be viewed or searched.
For long-term use, install Team Edition: the
mattermost-team-packages in the version archive, or the team-edition Docker image. - Team Edition has no message limit, but it has a user cap. An unlicensed server is hard-capped at 250 active users, with warnings from 200. Compiling from modified source removes it; the official builds do not.
- Single sign-on is paid. Since v11 Team Edition has dropped even GitLab login; SAML, AD/LDAP and Entra ID are in the paid Professional and Enterprise plans.
- Diagnostics reporting is on by default. Turn off "Enable Diagnostics and Error Reporting" under Environment > Logging in the System Console.
- Mobile push goes through the official Test Push Notification Service (TPNS) by default, which is unencrypted and has no delivery guarantee. The production push service requires a paid licence, or you build the apps and push proxy yourself.