Open-source alternatives to Slack

4 community-reviewed projects you can use instead of Slack.

1. Mattermost - Mattermost is an open-core, self-hosted team collaboration platform with channels, threads, direct messages, file sharing, searchable history and calls, deployable on your own infrastructure including private-cloud and air-gapped environments on web, desktop and mobile. (Licence: NOASSERTION)
2. Rocket.Chat - Rocket.Chat is an open-source, self-hostable communications platform combining team chat, voice and video calls, file sharing and omnichannel customer messaging in one deployment, with web, desktop and mobile apps and options for on-premises and air-gapped use. (Licence: NOASSERTION)
3. Zulip - Zulip is an open-source team chat app with topic-based threading that combines email-style organization with real-time messaging. It can be self-hosted on Linux/Docker or used via Zulip Cloud, with web, desktop and mobile apps. (Licence: Apache-2.0)
4. Element - Element is a free, open-source messaging and collaboration client built on the decentralized Matrix protocol, with end-to-end encryption, group chat, file sharing and voice/video calls. It works on web, desktop and mobile, and can be self-hosted or used on public Matrix servers. (Licence: AGPL-3.0)