Slack vs Mattermost vs Rocket.Chat – Best Open Source Team Chat
Closed-source team chat tools such as Slack are convenient, but they come with real costs: your message history lives on someone else’s servers, free tiers expire your data, and you have no way to self-host, customise, or move your workspace. For teams that care about data ownership, privacy, or long-term costs, open source team chat platforms are a serious alternative. Here’s how the two biggest — Mattermost and Rocket.Chat — compare with Slack, the proprietary default.
Mattermost
Mattermost is a self-hosted, MIT-licensed team messaging platform built as a drop-in Slack-style workspace. It excels at secure collaboration inside your own infrastructure, with deep integrations into the developer toolchain.
Strengths
- Self-hosted by default: run it on your own servers, in the cloud, or air-gapped — full control over your data.
- Unlimited message history: no 90-day retention wall; you own every message.
- Developer integrations: GitLab, GitHub, Jira, Jenkins and a full REST API plus webhooks and slash commands.
- Enterprise governance: compliance reports, audit logs, data spillage handling, and granular access control policies.
- Interoperability: recent releases add federated messaging via the Matrix protocol and an embedded option for Teams video calls.
The latest release as of this post is v11.11 (September 2026), with v12.0 planned for October 2026. Mattermost is a strong pick for regulated industries, DevOps teams, and anyone who wants a familiar Slack-like UI without surrendering data.
Rocket.Chat
Rocket.Chat is another MIT-licensed, self-hosted team communication platform, and one of the most widely deployed open source alternatives in the space. Beyond plain messaging it markets itself as a complete customer-facing communications hub.
Strengths
- Omnichannel support: live chat, email, and social messaging in one inbox — built for support teams, not just internal chat.
- Built-in audio and video: voice calls and screen sharing, now with general-availability screen-share controls.
- Enterprise directory integration: LDAP, Active Directory, SAML, and OAuth single sign-on for existing users.
- End-to-end encryption: force E2EE on private rooms so encryption can’t be skipped.
- AI-ready platform: version 8.8 adds a native Model Context Protocol (MCP) server so AI agents can work inside your workspace.
- Federation: connect rooms across servers via Matrix and now XMPP federation bridges.
The current stable release is 8.8.0 (September 2026). Note that from version 9.0.0 onward LDAP and SAML authentication will require a paid license, so budget accordingly if you depend on directory sign-in.
Slack — the proprietary baseline
Slack is the incumbent and the UX benchmark. Its strengths are polish, a huge app ecosystem, and zero setup. But on the free plan the trade-offs stack up quickly:
- Only the most recent 90 days of message and file history are searchable.
- Slack permanently deletes messages and files older than one year on free workspaces.
- Integrations are limited to 10 apps.
- Group huddles, unlimited history, SAML SSO, and workflow automation are locked behind paid plans (roughly $8–15 per user per month).
Slack is proprietary with no self-hosting option and no open source license. If you want to keep your data, avoid subscription creep, or run a fully sovereign stack, Mattermost and Rocket.Chat both give you the same core experience for free.
Side-by-side comparison
| Feature | Mattermost | Rocket.Chat | Slack (Free) |
|---|---|---|---|
| License | Open source (MIT) | Open source (MIT) | Proprietary |
| Self-hosted | Yes | Yes | No |
| Message history | Unlimited | Unlimited | 90 days, then deleted |
| Video / voice calls | Yes (via integrations) | Built-in + screen share | Limited (1:1 huddles) |
| Omnichannel / support hub | Limited | Built-in | Paid add-ons |
| LDAP / SSO | Yes | Yes (license from v9) | Paid |
| Federation | Matrix protocol | Matrix + XMPP | No |
| Price | Free (self-hosted) | Free (self-hosted) | Free tier / paid plans |
Which should you choose?
Start with Mattermost if your priority is a familiar, secure Slack-like workspace in front of your own data, with strong developer tool integrations and straightforward compliance tooling — especially for engineering and DevOps teams.
Choose Rocket.Chat if you also need a customer-facing support platform — omnichannel inbox, built-in video calls, and powerful federation — and you want one self-hosted tool that spans both internal chat and external engagement. Just plan for the upcoming LDAP/SAML licensing change if you rely on directory sign-in.
Stick with Slack only if you need the commercial app ecosystem, don’t mind a subscription, and are comfortable with a capped free tier and third-party data.
Both open source contenders are free, fully self-hosted, and let you own your conversation history. You can read the full details on the Mattermost listing and the Rocket.Chat listing, or browse hundreds more free and open source tools in the directory. Know a great team chat platform we’re missing? Suggest it and we’ll add it.