The Best Open Source Alternatives to Slack

Mattermost as a Slack alternative

Mattermost details

Mattermost is the closest like-for-like replacement for Slack: channels, threads, and direct messages laid out the same way, an import tool that ingests a Slack export, and desktop and mobile apps on every platform Slack ships to. It keeps the parts teams leave Slack over — message history is never truncated by a plan tier, and the whole workspace runs on infrastructure you control.

Rocket.Chat as a Slack alternative

Rocket.Chat details

Rocket.Chat covers Slack's ground and then keeps going: alongside channels and threads it does end-to-end encrypted rooms, federation with other Matrix and Rocket.Chat servers, and a customer-facing livechat widget. That last piece matters if Slack Connect is how you talk to people outside the company — Rocket.Chat handles internal chat and external conversations in one deployment.

Zulip as a Slack alternative

Zulip details

Zulip disagrees with Slack about how conversation should work, and that is the reason to pick it. Every message belongs to a named topic inside a channel, so a busy channel stays readable and catching up after a day away is a matter of reading the topics that concern you rather than scrolling a merged firehose. Teams that find Slack unmanageable at volume tend to be the ones Zulip suits best.

Element as a Slack alternative

Element details

Element is the option for teams whose objection to Slack is that Slack can read the messages. It is a client for Matrix, an open federated protocol, with end-to-end encryption on by default rather than as an enterprise add-on. Federation also means your server can talk to other Matrix servers, so contractors and partners keep their own homeserver instead of becoming guests on yours.