Slack is team chat with channels, threads, search, integrations and calls. If you need the data on your own servers, or you want to stop paying per seat, several open-source platforms cover the same ground with different ideas about how conversations should work.
Every tool below has a repository licence we checked: AGPL-3.0, Apache-2.0 or MIT. Several are open-core, meaning some enterprise features sit outside the open-source code, and we call that out where a project says so. Mattermost's licence has a particular quirk worth reading in its entry.
Licences checked against each repository on November 7, 2026.
Best for: Async teams that want topic-based threading
Describes itself as open-source team chat with topic-based threading that combines the best of email and chat, designed for both live and asynchronous conversation. Self-host on Ubuntu or Debian, with Docker, or use prebuilt images.
Strengths
+ Topic threading keeps busy channels readable
+ Apache-2.0 licence
+ Several self-hosting routes
Trade-offs
− Topic-based model differs from Slack's channel flow
− Paid cloud plans exist, the README does not list feature limits
Best for: Customisable chat for strict data-protection needs
Describes itself as an open-source, secure, fully customisable communications platform. Lists end-to-end encryption, role- and attribute-based access control, federation, an Apps-Engine with a marketplace, and deployment via Docker, Podman or Kubernetes, including air-gapped setups.
Strengths
+ Strong access-control and encryption features
+ Apps-Engine for custom integrations
+ Air-gapped deployment option
Trade-offs
− Repository includes an ee directory, check which features need a paid plan
− Premium hosting is sold alongside the open-source edition
Best for: Self-hosted chat plus workflow automation
Describes itself as an open core, self-hosted collaboration platform with chat, workflow automation, voice calling and screen sharing. Runs as a single Linux binary on PostgreSQL, with Docker, Kubernetes and Helm install guides and native desktop and mobile clients.
Strengths
+ Single binary on PostgreSQL
+ Large integration marketplace
+ Native clients on desktop and mobile
Trade-offs
− Licence is layered: source under AGPL-3.0 or commercial, official compiled builds under MIT, some parts under Apache-2.0
− A separate enterprise licence file covers paid features
Synapse is an open-source Matrix homeserver written and maintained by Element. Matrix is described as an open standard for secure and interoperable real-time communications, so your server can federate with others. Element Web is the usual client.
Strengths
+ Federation lets organisations interoperate
+ Open standard rather than a single vendor's product
+ Backed by Element, with Element Server Suite deployment
Trade-offs
− Also dual-licensed under a paid Element Commercial License
− Element provides no support without a subscription
The backend services that power the Stoat chat service. Written in Rust, with a REST API server, a WebSocket events server, file and proxy servers, and voice via LiveKit. The stack uses MongoDB, Redis and MinIO.
Strengths
+ Open backend you can run yourself
+ Includes voice support
+ Separate web client repository
Trade-offs
− README documents a development setup, not production hosting
Best for: Chat and calls if you already run Nextcloud
A chat and video and audio conferencing app for Nextcloud. Offers private, group and public calls, screen sharing, federated chats and integration with Files, Calendar and other Nextcloud apps.
Strengths
+ Fits neatly into an existing Nextcloud
+ Calls, chat and file sharing together
+ Federated chats across Nextcloud servers
Trade-offs
− Requires a Nextcloud server
− Behind strict firewalls you may need a TURN server
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How to choose
If you need…
Start with
Async-friendly team chat with strong threading
Zulip
Strict security controls or air-gapped hosting
Rocket.Chat
Chat with built-in workflow automation
Mattermost
Federation with other organisations
Matrix Synapse
You already run Nextcloud
Nextcloud Talk
A community-style chat with voice
Stoat
Open-source Slack alternatives: FAQ
Quick answers to common questions
What is the best open-source alternative to Slack?
Mattermost and Rocket.Chat are the closest to Slack's channel model, while Zulip takes a different approach with topic threading. Matrix Synapse is the pick when federation matters.
Is Mattermost open source?
Mostly. Its repository describes the source as AGPL-3.0, with a commercial option, and official compiled builds as MIT. It calls itself open core and keeps a separate enterprise licence file, so read LICENSE.txt before committing.
Which Slack alternative is easiest to self-host?
Mattermost describes itself as a single Linux binary relying on PostgreSQL, and Zulip lists Ubuntu or Debian, Docker and prebuilt images. Rocket.Chat deploys with Docker, Podman or Kubernetes.
Can I import Slack history?
Some tools provide Slack importers, but we have not verified them here. Check each project's docs before planning a migration.
Is Matrix the same as Element?
No. Matrix is the open protocol, Synapse is a server that implements it, and Element is a client. Both Synapse and Element Web are AGPL-3.0 with an optional commercial licence.
Maintain an open-source Slack alternative?
List it on TechLogHub and we'll review it for this page.
