# 1 Best Open-Source Alternatives to Slack in 2026

By Noah — last updated & pricing verified September 15, 2026.

> 1 of the 5 Slack alternatives we list qualify for this filter. The [full list](https://findalternative.to/slack) is the canonical page.

| Alternative | Best for | Price | Details |
| --- | --- | --- | --- |
| Mattermost | regulated, government, or defense organizations that need self-hosted, air-gapped, or sovereign-cloud team chat with compliance controls a SaaS-only tool like Slack can't provide | Free | https://findalternative.to/mattermost/about |

All 5 alternatives to Slack: https://findalternative.to/slack

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Markdown mirror of https://findalternative.to/slack/open-source — findalternative.to, honestly ranked software alternatives. AI info: https://findalternative.to/ai