4 open source alternatives to Slack
Team messaging with channels, threads, and integrations. Here are the open source projects real teams use instead — ranked by fit, with honest pros and cons for each.
What people don't love about Slack
- Free tier message history cap makes it unusable for serious teams.
- Data residency concerns for EU and regulated industries.
- Per-seat pricing scales expensively.
Current Slack pricing (for reference): Free tier limited to 90 days of history; Pro from $7.25/user/month.
Quick comparison
| Alternative | Best for | License | Self-host | Hosted cloud? |
|---|---|---|---|---|
|
Mattermost Slack-style open source team messaging. |
The most drop-in Slack replacement for teams. | AGPL-3.0 / Apache-2.0 (modules) | ★★★☆☆ | Yes |
|
Rocket.Chat Open source team and community chat with video and voice. |
Large teams that want omnichannel and federation. | MIT (community); proprietary modules available | ★★★☆☆ | Yes |
|
Zulip Threaded team chat with topics. |
Async-heavy teams who like threaded topics. | Apache-2.0 | ★★★☆☆ | Yes |
|
Element (Matrix) Decentralized messaging on the Matrix protocol. |
When end-to-end encryption is a hard requirement. | Apache-2.0 | ★★★★☆ | Yes |
1. Mattermost — The most drop-in Slack replacement for teams.
Slack-style open source team messaging.
Strengths
- Familiar Slack-like UX.
- Strong self-hosting story and enterprise features.
- Good permission and compliance controls.
Weaknesses
- Some advanced features require paid tier.
- Mobile app performance is acceptable but not stellar.
- Larger install footprint than lightweight chats.
Mattermost homepage · Source on GitHub · Slack vs Mattermost →
2. Rocket.Chat — Large teams that want omnichannel and federation.
Open source team and community chat with video and voice.
Strengths
- Federation support (via Matrix bridge).
- {'Large feature set': 'channels, threads, omnichannel.'}
- Strong customization options.
Weaknesses
- Can feel heavy for small teams.
- Past performance issues on large deployments.
- Resource footprint is higher than competitors.
Rocket.Chat homepage · Source on GitHub · Slack vs Rocket.Chat →
3. Zulip — Async-heavy teams who like threaded topics.
Threaded team chat with topics.
Strengths
- Topic-based threading prevents channel noise.
- Great for async and open-source communities.
- Powerful search and history.
Weaknesses
- Topic model has a learning curve.
- Smaller ecosystem of third-party integrations.
- UI feels less polished than Slack.
4. Element (Matrix) — When end-to-end encryption is a hard requirement.
Decentralized messaging on the Matrix protocol.
Strengths
- Fully federated — you own your data.
- End-to-end encryption by default.
- Bridges to Slack, Discord, WhatsApp, etc.
Weaknesses
- Self-hosting Synapse or Conduit server is work.
- E2E encryption UX (device verification) can confuse users.
- Cross-signing and key backup setup is fiddly.
Element (Matrix) homepage · Source on GitHub · Slack vs Element (Matrix) →
Not what you're looking for?
Browse other tools in Team Communication, or check out open source projects by category on the full category index.
Recommended reading
Why your team probably can't ditch Slack yet (and what needs to change)
Mattermost and Rocket.Chat are excellent. So why do most teams who try to migrate away from Slack end up back on Slack? An honest look at the real blockers.
When self-hosting goes wrong: seven failure modes and how to avoid them
An honest retrospective on the ways self-hosted setups break — not in theory, but in practice — and the small habits that prevent most of them.
Will the open source project you depend on still exist in three years?
Bus factor, maintainer burnout, funding models, and the signals that separate OSS projects that survive from those that quietly decay.