Mattermost
Secure team collaboration — messaging, playbooks, and integrations.
MIT-licensed alternative to Slack with channels, threads, webhooks, Jira/GitHub integrations, and Playbooks for incident response. Desktop and mobile apps.
Rocket.Chat
Self-hosted team messaging — channels, video, and compliance.
MIT-licensed alternative to Slack and Mattermost with end-to-end encryption, Omnichannel, apps marketplace, and FedRAMP-ready compliance.
Zulip
Topic-threaded team chat — keeps conversations organized at scale.
Zulip is an open-source team chat application built around topic-based threading, which keeps conversations organized in a way that channel-only tools like Slack or Rocket.Chat don't. Apache-2.0 licensed and self-hostable.
Element
Matrix-based team messaging — decentralized and end-to-end encrypted.
Element is an open-source, end-to-end encrypted collaboration client built on the decentralized Matrix protocol, so it can federate across independent servers rather than relying on one company's infrastructure. AGPL-3.0 licensed.
Campfire
Simple self-hosted group chat from 37signals — one-time, MIT-licensed, no subscription.
Campfire is a simple, self-hosted group chat application from 37signals, released as open source under the MIT license in 2025. It's deliberately minimal — @mentions, DMs, rooms — with no recurring subscription.
Wire
End-to-end encrypted team collaboration — messaging, calls, EU-based.
Wire is an open-source, end-to-end encrypted collaboration platform for messaging, calls, and file sharing, aimed at organizations with strong security and compliance requirements. AGPL-3.0 licensed, based in the EU.
Fluxer
Discord-style chat for communities — messaging, voice, and video, self-hostable.
Fluxer is a free, open-source instant messaging and VoIP platform for friends, groups, and communities, styled after Discord. AGPL-3.0 licensed with self-hosting support.
Cinny
Elegant Matrix client — end-to-end encrypted, clean interface.
Cinny is an open-source Matrix client focused on a simple, elegant interface for end-to-end encrypted conversations. AGPL-3.0 licensed, it connects to any Matrix homeserver.
Stoat
Self-hostable Discord alternative (formerly Revolt) — servers, channels, voice.
Stoat (formerly Revolt) is an open-source, self-hostable group chat platform styled after Discord — servers, channels, voice, and moderation — built in the EU with no ads or data mining. Self-hosted components are AGPL-3.0 licensed.
Quiet
Peer-to-peer team chat over Tor — no servers, no phone number, no metadata.
Quiet is an open-source, peer-to-peer team chat that syncs messages directly between devices over Tor, with no central server, no phone number, and no metadata collection. GPL-3.0 licensed.
Compare Slack alternatives
| Tool | Stars | Forks | License | Self-hosted |
|---|---|---|---|---|
| Mattermost | 36.9K | 8.7K | MIT | Yes |
| Rocket.Chat | 45.4K | 13.6K | MIT | Yes |
| Zulip | 25.5K | 9.9K | Apache-2.0 | Yes |
| Element | 13.3K | 2.7K | AGPL-3.0 | Yes |
| Campfire | 4.4K | 747 | MIT | Yes |
| Wire | 2.8K | 333 | AGPL-3.0 | Yes |
| Fluxer | 9.5K | 632 | AGPL-3.0 | Yes |
| Cinny | 3.8K | 551 | AGPL-3.0 | Yes |
| Stoat | 3.1K | 367 | AGPL-3.0 | Yes |
| Quiet | 2.6K | 141 | GPL-3.0 | Yes |
Slack is a proprietary team messaging platform with per-seat pricing that scales expensive as teams grow, and message history that lives on Slack's servers. Open-source alternatives let you self-host, avoid per-seat fees, and keep your conversation data under your own control.
They're not interchangeable, though — the right pick depends on whether you need a drop-in team messenger, async-friendly threading, end-to-end encryption, or something deliberately simpler.
The best open-source alternatives to Slack
1. Mattermost — best all-in-one Slack replacement
Mattermost is the closest overall substitute for Slack: channels, threads, direct messages, calls, and integrations in a single self-hosted server, with workflow "playbooks" that structure incident response and recurring processes. Its center of gravity is technical teams — DevOps and engineering orgs are its core audience.
MIT-licensed core, with some enterprise capabilities in paid tiers. For most teams leaving Slack, this is the safest default migration path.
Why pick it over Slack:
- Familiar channels-and-threads model — minimal retraining
- Self-hosted: your message history stays on your infrastructure
- Playbooks for incident response and repeatable workflows
- Deep integrations with developer tooling
- MIT-licensed core, no per-seat fee when self-hosted
2. Rocket.Chat — best for breadth
Rocket.Chat covers a wider surface than most alternatives: team messaging plus omnichannel customer conversations (support inboxes from web chat, email, WhatsApp), built-in voice and video, and compliance features for regulated environments — under an MIT-licensed core.
Pick it when chat is also part of how you talk to customers, not just teammates, or when air-gapped/zero-trust deployment requirements rule out cloud tools entirely.
Why pick it over Slack:
- Omnichannel customer messaging alongside team chat
- Built-in voice, video, and AI features
- Air-gapped and zero-trust deployment options
- MIT-licensed with a large self-hosting community
- Marketplace of apps and integrations
3. Zulip — best for async, high-volume teams
Zulip's topic-based threading is a genuinely different conversation model: every message belongs to a channel and a topic, so a busy team's discussions stay organized and catch-up-able instead of scrolling away. Distributed teams and open-source communities are where it shines.
Apache-2.0 licensed and battle-tested (it powers many large open-source projects' communities). The cost is a mental-model shift for people used to Slack's linear channels.
Why pick it over Slack:
- Topic threading keeps high-volume discussion readable
- Built for asynchronous work across time zones
- Apache-2.0, self-host at no license cost
- Excellent public-community features (open archives, moderation)
- Full-text search that actually finds things
4. Element — best for federation and end-to-end encryption
Element is the flagship client for Matrix, an open, decentralized protocol: independent servers federate with each other (closer to email than to a walled garden), and conversations are end-to-end encrypted by default. Organizations run their own homeserver for full data sovereignty.
The price of that openness is operating Matrix infrastructure and accepting protocol-level complexity Slack never exposes. AGPL-3.0 licensed.
Why pick it over Slack:
- End-to-end encryption on by default
- Federated: message users on other Matrix servers seamlessly
- Full data sovereignty via your own homeserver
- Interoperates with any Matrix client (like Cinny)
- Voice and video included
5. Campfire — best minimal option
Campfire is 37signals' deliberately simple group chat — rooms, direct messages, @mentions, search, file sharing — released as open source under MIT in 2025 after starting life as a paid product. It's a single Ruby on Rails app designed to run on one server.
For a small team that wants a chat server it can read end to end and own outright, that restraint is the feature. It will never grow Slack's surface area, by design.
Why pick it over Slack:
- The entire codebase is small enough to actually audit
- One-server deployment, minimal operational overhead
- MIT-licensed, no subscription of any kind
- Covers the 90% of chat teams actually use
6. Wire — best for compliance-heavy organizations
Wire applies end-to-end encryption across messaging, voice, video, and file sharing by default, and is developed in the EU under strict data-protection law — aimed squarely at regulated industries, government, and security-conscious enterprises.
AGPL-3.0 licensed server and clients, self-hosted or on a managed EU cloud.
Why pick it over Slack:
- E2E encryption as the default, not an option
- EU jurisdiction and GDPR-first posture
- Enterprise compliance controls
- Self-hostable, AGPL-3.0
Also worth a look depending on your use case: Fluxer and Stoat if what you actually want is Discord-style community chat, Cinny as a lighter Matrix client, and Quiet for a serverless, peer-to-peer approach over Tor.
FAQ
Is there a free, open-source alternative to Slack?
Yes. Rocket.Chat, Mattermost, Zulip, and Campfire are all permissively licensed and free to self-host, with no per-seat fees for the self-hosted version.
What's the best open-source alternative to Slack?
Mattermost for a direct all-in-one replacement. Zulip if topic-threaded, asynchronous discussion suits your team better than channels alone.
Which Slack alternatives are end-to-end encrypted?
Element and Wire provide end-to-end encryption by default, and Quiet uses a serverless peer-to-peer architecture over Tor for maximum metadata privacy.
What are the top open-source alternatives to Slack?
Browse all open-source alternatives to Slack above, or explore the Communication & Community category.