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 four leading contenders — Mattermost, Rocket.Chat, Zulip and Element — compare with Slack, the proprietary default.

Mattermost

Mattermost is a self-hosted, Slack-style team messaging platform built as a drop-in workspace. Its source is released under AGPL-3.0 (official compiled builds ship under an MIT licence), so you can inspect, modify and run it yourself. 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 self-hosted team communication platform, and one of the most widely deployed open source alternatives in the space. Its core is MIT-licensed, with enterprise-only modules under a separate licence. 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.

Zulip

Zulip is an Apache-2.0-licensed, self-hostable team chat platform whose defining feature is topic-based threading: every conversation is its own thread inside a stream, so you can follow, mute and catch up on each one independently. Teams that struggle to keep up with busy Slack channels often find that model easier to reason about than one endless scrolling timeline.

Strengths

• Topic-based threading: every conversation is a thread you can follow, mute or catch up on independently, instead of one continuous channel scroll.
• Permissive licence: the whole server is Apache-2.0 — the most permissive licence in this line-up — and fully self-hostable.
• Powerful search: full-text search across the entire message history, with saved filters.
• Apps everywhere: web, desktop (Linux, macOS, Windows), iOS and Android clients.
• Integrations and imports: native GitHub and GitLab bots, a Slack import tool, Zapier, and a rich REST API with webhooks.
• Video calls: built-in integrations for Zoom, Jitsi and BigBlueButton.

Element (Matrix)

Element takes a different approach: it is the flagship client for the Matrix protocol — an AGPL-3.0-licensed, end-to-end encrypted messenger that is decentralised rather than a single hosted service. You can use a public homeserver, or point Element at your own self-hosted server (such as Synapse) so your conversations never leave your infrastructure.

Strengths

• End-to-end encryption by default: secured with cross-signing and encrypted key backups, so you don’t lose access to your own history.
• Federated and decentralised: no single company or server owns the network; you choose which homeserver your data lives on.
• Self-hosting friendly: run your own Synapse or Conduit homeserver for full data control.
• Bridges to other networks: Matrix bridges connect Element to platforms such as Slack, Discord, IRC and XMPP, so you can migrate gradually or keep talking to partners on other networks.

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, Rocket.Chat, Zulip and Element all give you the same core experience for free.

Side-by-side comparison

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.

Pick Zulip if your team lives in long, fast-moving discussions and you want each conversation to stay separate. Its topic-based threading is the strongest answer to channel overload, and Apache-2.0 is the most permissive licence here for self-hosting.

Choose Element if sovereignty and encryption matter most: Matrix is federated, end-to-end encrypted by default, and lets you run your own homeserver while still bridging to the networks your partners already use.

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.

All four open source contenders are free, fully self-hosted, and let you own your conversation history. You can read the full details on the Mattermost, Rocket.Chat, Zulip and Element listings, 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.