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.