5 Best Open Source Slack Alternatives

Slack is a workplace messaging platform organized around channels, threads, and direct messages, with a large ecosystem of app integrations that connect it to other business tools. It became the default team chat app for many companies, particularly in tech, thanks to its search, notification handling, and third-party integrations.

Organizations look at open-source alternatives for cost and data control reasons that grow with company size. Slack pricing is per active user, and message history retention on lower tiers has historically been limited, pushing teams toward paid plans as chat becomes core to how they operate. Message and file data lives on Slack's servers, which raises questions for organizations with strict data residency requirements or a preference for keeping internal communication entirely in-house.

Rocket.Chat, Mattermost, and Element are the main self-hosted contenders. Rocket.Chat and Mattermost both closely follow Slack's channel-and-thread model and support similar integrations, bots, and search. Element, built on the Matrix protocol, works differently by federating across independently run servers, which matters if you want to communicate with other organizations without everyone joining the same instance.

Before switching, check integration support for the specific third-party tools your team relies on daily, since Slack's app directory is larger than any self-hosted alternative's plugin ecosystem. Test mobile app performance and notification reliability under real use, as these are common weak points for self-hosted chat. Consider whether you're prepared to run and secure the server infrastructure yourself, or whether a managed hosting plan from the same open-source vendor is worth paying for to skip that operational overhead while keeping data off Slack's platform specifically.