Open-source team chat apps and Slack alternatives

Team chat platforms you can run on your own servers, so company conversations, files and history stay under your control instead of a vendor's.

3 open-source projects, checked against GitHub on 1 November 2026.

All 3 team chat projects

Showing all 3, ordered by health score.

  1. 92 Active
    Zulip

    Zulip server and web application. Open-source team chat that helps teams stay productive and focused.

    Apache-2.0
    Setup
    Docker
    Stars
    26.2k
    Release
    21 September 2026
  2. 82 Active
    Rocket.Chat

    The Secure CommsOS™ for mission-critical operations

    MIT paid features in repoCLA
    Setup
    3 of 5
    Stars
    46.4k
    Release
    5 October 2026
  3. 74 Active
    Mattermost

    Mattermost is an open source platform for secure collaboration across the entire software development lifecycle..

    Apache-2.0 paid features in repo
    Setup
    5 of 5
    Stars
    39.5k
    Release
    22 October 2026

Our picks by use case

  • Conversations sorted by topic

    Zulip puts every message in a topic inside a channel, so people can catch up asynchronously, and the whole repository is Apache-2.0.

    • ZulipApache-2.0health 92

  • Chat for engineering and security teams

    Mattermost adds workflow automation, calls and screen sharing, with enterprise features under a separate license.

    • MattermostApache-2.0paid features in repohealth 74

  • Talking to customers and partners too

    Rocket.Chat supports federation, end-to-end encryption and installations without internet access, for organizations with strict data rules.

How to choose a team chat app

The main differences are how conversations are organized, what sits behind a paid license and how heavy the setup is. Channel-based apps feel like Slack, while topic-based threading suits teams that read messages hours later.

Channels or topics

In a channel-based app, a busy channel becomes one long stream that is hard to read later. Topic threads keep parallel discussions apart, which helps distributed teams but takes a few days to get used to.

Open source or open core

Mattermost and Rocket.Chat keep some features in enterprise directories under paid licenses, while Zulip has no such split. List the features you need and check whether they belong to the paid edition before you plan a rollout.

Running it

Mattermost runs in production as one binary with PostgreSQL, Rocket.Chat's compose file starts six services, and Zulip installs on Ubuntu, Debian or Docker. Chat history quickly becomes important, so plan backups and updates from the first day.

Moving from Slack or Teams

Test the desktop and mobile apps, notifications and sign-in with a small group first. Agree on how channels or topics will be named, because that structure is hard to change once hundreds of people use it.

Drafted with AI from the projects’ READMEs and our data, then checked against that data on 31 October 2026. Report a correction

Which of these are really open source?

Fully open source (1)OSI-approved license, no paid parts in the repository
Zulip
Open source with paid parts (2)OSI license, but some features are under commercial terms
Rocket.Chat, Mattermost
Not OSI open source (0)Code is public, use is restricted
None

Read automatically from each repository’s license files every day. How we read licenses

Questions about open-source team chat

What are the most popular open-source team chat projects?

By GitHub stars: Rocket.Chat (46.4k), Mattermost (39.5k), Zulip (26.2k). By our health score, the most actively maintained are Zulip (92), Rocket.Chat (82), Mattermost (74).

Which team chat projects are fully open source?

1 of the 3 use an OSI-approved license with no paid parts in the repository, including Zulip.

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