Open source Slack alternatives: Mattermost, Rocket.Chat, Zulip and Element

Teams usually leave Slack for one of three reasons: message history on the free plan disappears, per-seat pricing gets expensive as the company grows, or the organization needs its conversations on servers it controls for legal or security reasons. All four apps below can run on your own server, have desktop and mobile apps, and can import at least some of your Slack history. They differ most in how they organize conversation, and in what the company behind each one keeps for its paid plans. We read each license and latest release on November 5, 2026.

Quick comparison

Chat app	License	Feels like	Mobile apps	Best for
Mattermost	AGPL-3.0 source, MIT compiled builds	Slack, channel for channel	Yes	Engineering and security-sensitive teams
Rocket.Chat	MIT (core), enterprise code separate	Slack plus a customer chat inbox	Yes	Teams that also talk to customers
Zulip	Apache-2.0	Email threads inside chat	Yes	Distributed and asynchronous teams
Element (Matrix)	AGPL-3.0	Slack, on a federated network	Yes	Encryption and talking across organizations

Mattermost: the closest thing to self-hosted Slack

Mattermost looks and behaves the most like Slack: public and private channels, threads, direct messages, emoji reactions, slash commands, incoming and outgoing webhooks, and a plugin system. It is popular with engineering, DevOps and government teams, and it supports structured workflows called Playbooks for incident response. It runs on Docker, Kubernetes or a plain Linux server with PostgreSQL.

The licensing is layered. You can build the source yourself under AGPL-3.0, and Mattermost publishes compiled releases under the MIT license every month. Features such as advanced compliance exports and some single sign-on options belong to paid editions, so compare the current editions against your requirements before you commit. Version 12.0.0 shipped on October 14, 2026.

Rocket.Chat: team chat and customer conversations together

Rocket.Chat covers internal chat and also an omnichannel inbox, so your support team can answer website chat, email and messaging apps in the same tool your staff use to talk to each other. It supports end-to-end encrypted rooms, federation with other servers, an app marketplace, and air-gapped installs for networks without internet access.

Code outside its ee/ directories is MIT licensed; enterprise features in those directories are under a separate license. Version 8.9.0, a long-term support release, shipped on October 5, 2026. Rocket.Chat is the most feature-packed of the four, which also makes it the heaviest to administer. Choose it when chat with customers matters as much as chat between colleagues.

Zulip: threads that make catching up easy

Zulip organizes every message into a channel and a topic, like an email subject line. Instead of scrolling through one fast-moving channel, you read conversation by conversation, mark a topic as read, and reply in the right place even hours later. Teams spread across time zones, open source projects and research groups often find this calmer than Slack, although it takes a week or two to get used to.

The whole server is Apache-2.0 licensed, with no separate enterprise code in the repository. Zulip publishes installers for Ubuntu and Debian and an official Docker image. Version 12.3 was the latest release on September 21, 2026. The company sells hosting, and offers free or discounted cloud plans to many open source projects and nonprofits.

Element: chat on the Matrix network

Element is the best-known app for Matrix, an open standard for decentralized, end-to-end encrypted chat. Your server, usually Synapse, joins a federated network, so people at different organizations can share rooms while each keeps their own server, much as email works. Bridges connect Matrix to other chat networks. Element is used by several European governments and armed forces for exactly these reasons.

Element Web and the Synapse server are AGPL-3.0, also available under a commercial license from Element. Element Web 1.12.33 shipped on October 27, 2026. Running Synapse well takes more care than the other three, especially as rooms grow, so read the server documentation before you invite the whole company. For mobile, Element now recommends its newer Element X apps.

How to choose

• You want the least retraining from Slack: Mattermost.
• You also want to answer customers in the same tool: Rocket.Chat.
• Your team works across time zones and drowns in channels: Zulip.
• You need encryption by default or chat with other organizations: Element on Matrix.

Before you migrate, export your Slack workspace and test the import on a copy, because file attachments, private channels and user mapping behave differently in each tool. Moving off Discord instead? See our Discord alternatives. You can browse every project in the directory.