What is it?
Mattermost is team chat you run on your own servers, laid out like Slack: teams, public and private channels, direct messages and threads. The server is written in Go and the web app in React, and the whole thing ships as a single Linux binary backed by PostgreSQL, installed with Docker, a tarball, Kubernetes or Helm. Native apps cover Android, iOS, Windows, macOS and Linux.
Mattermost describes itself as an "open core" platform for defense, intelligence, security and critical infrastructure organizations, and its README leads with DevSecOps, incident resolution and IT service desk use cases. For the same job, Rocket.Chat is written in TypeScript, while OpenHive is a Next.js and Supabase project started in March 2026 that has not published a release yet. The comparison of open-source Slack alternatives weighs the caps on Mattermost's free editions against the other tools.
Why is it interesting?
- Integrations: webhooks, slash commands, an API, bots and server-side plugins, with more than 700 integrations listed in the marketplace.
- Playbooks: a team procedure such as incident response kicks off on a keyword in a channel and walks through its tasks and automated actions.
- Built-in calls: audio calls with screen sharing from the browser, the desktop app or a phone, each with its own chat thread.
- Channels shared across organizations: connected workspaces sync a channel's messages, reactions and files between several Mattermost instances. The docs limit them to the Entry and Enterprise editions.
- Extended support releases: a new version lands on the 16th of every month, and an ESR every nine months, patched for twelve months for teams that upgrade rarely.
Good to know
The license has layers. The Team Edition binary built by Mattermost, Inc. is MIT; the Enterprise Edition binary is under a commercial license. The source code is under AGPL-3.0 or a commercial license, so a fork that a third party compiles stays AGPL. The webapp/ folder (the user interface) and the configuration files are under Apache-2.0. The server/enterprise folder falls under the "Mattermost Source Available License": you may modify it, but production use requires an Enterprise subscription. Rocket.Chat uses the same arrangement for its ee/ folders and releases the rest of its code under MIT.
Two free editions, sized for different teams. Team Edition targets fewer than 250 activated users and has no SSO. Since v11 (October 2025) its cap dropped from 1,000 to 250 users and GitLab SSO was removed, and the docs advise against it for government or sensitive workloads. Entry is the free mode of the Enterprise binary. It includes SAML, Entra ID and OpenID SSO, but it is recommended for under 50 users, has no high availability or compliance features, and shows only 10,000 messages of history across the whole server. Older messages stay in the database but can no longer be viewed or searched. More caps are announced for a future release: 40-minute calls, 5 playbook runs, 250 AI agent queries and 10,000 push notifications per month.
What the paid plans hold back. They lift the Entry caps and add commercial support. Group calls of up to 50 concurrent users require Professional, Enterprise or Cloud. High availability clustering, Elasticsearch or OpenSearch search, compliance export and message retention policies start at the Enterprise plan. Professional is meant for up to 250 users. No prices are listed: the pricing page sends you to sales. Mattermost's hosted Cloud runs the Enterprise edition.
What you need to run it. PostgreSQL 14 or later; MySQL support has been deprecated since v11. On a single server, the docs size 1 vCPU and 2 GB of RAM for up to 1,000 users, and 2 vCPUs and 4 GB for up to 2,000, on a 64-bit x86 processor. Built-in search runs in PostgreSQL and is meant for fewer than 2 to 3 million posts and files; past that, the docs recommend Elasticsearch or OpenSearch, which means the Enterprise plan.
Version 12 is due on 14 October 2026. The latest stable release is 11.11.1, published on 24 September 2026. v12.0 has been in release candidate since 18 September; it will require PostgreSQL 15 and drop support for RHEL 7 and 8. The current extended support release is 11.7, out on 15 May 2026 and maintained until 15 May 2027.