Open-Source Slack Alternatives (2026, Self-Hosted)

Identifying Slack · Mark source

forkalt Slack-class job graphic listing Zulip and Mattermost — forkalt comparison graphic (2026-10-23). Source · License: forkalt original.

People look for a Slack alternative when they want self-hosted team chat, an OSI-style license, or topic threading instead of channel-only chat. This hub lists tools we verified on 2026-10-23. Slack is named only to identify that product class.

Quick take.

Zulip — Apache-2.0 topic-threaded team chat you can self-host or run on Zulip Cloud.
Mattermost — Open-core self-hosted team collaboration. Mixed LICENSE.txt (compiled MIT, source AGPL).

How we pick

Public GitHub repository we could open (README or API).
License copied from LICENSE / LICENSE.txt, including mixed/open-core cases.
Self-hostable team chat documented by the project.
No unsourced “beats Slack” claims.

Comparison

Name	License	Stars	Last release	Platforms	Runs locally	Key strengths
Zulip	Apache-2.0	26,095	12.3	Linux, Docker, Web, Windows, macOS, Android, iOS	Yes	Apache-2.0 topic-threaded team chat you can self-host or run on Zulip Cloud.
Mattermost	other	39,410	v12.0.1	Linux, Docker, Kubernetes, Web, Windows, macOS, Android, iOS	Yes	Open-core self-hosted team collaboration. Mixed LICENSE.txt (compiled MIT, source AGPL).

Two Slack-class jobs: Zulip for topic-threaded chat, Mattermost for Slack-shaped channels — forkalt comparison graphic (2026-10-23). Not a ranking.

Listed options: Zulip (Apache-2.0, topic threading) and Mattermost (open-core, Slack-shaped). Identification only: Slack is Salesforce’s hosted team chat. We did not copy Slack pricing or a feature scorecard.

Sourced snapshot (2026-10-23)

Field	Zulip	Mattermost
Repository	github.com/zulip/zulip	github.com/mattermost/mattermost
Stars / forks	26,095 / 10,457	39,410 / 9,068
License	Apache-2.0 (`LICENSE`)	Mixed (`LICENSE.txt`; GitHub NOASSERTION)
Latest release in this check	Server 12.3 (2026-09-21)	v12.0.1 (2026-10-22)
Job	Streams + topics	Channels / open-core collaboration
Self-host	Ubuntu/Debian, Docker	Docker, tar, k8s, Omnibus
Quality vs Slack	Not independently tested	Not independently tested

Why Zulip is the OSI row

“Open source Slack alternative” is a durable query. The OSI-simple row we could source on 2026-10-23 from a public GitHub LICENSE is Zulip (zulip/zulip): Apache-2.0, 26,095 stars, Server 12.3. Its README’s job is topic-based threading, not a proof that it substitutes for Slack’s connector catalog, huddles, or Enterprise Grid.

Mattermost is the higher-star, Slack-shaped option (39,410 stars, v12.0.1). We list it because people search it by name. We do not collapse LICENSE.txt into “MIT.” Compiled vendor binaries are MIT; source you compile is AGPL v3.0 or commercial; server/enterprise is source-available. Same honesty pattern as n8n on Zapier alternatives.

Who should switch

Need an Apache-2.0 topic-threaded server: Zulip 12.3 from zulip.com/self-hosting.
Need Slack-shaped channels and accept mixed/open-core licensing: Mattermost v12.0.1. Read LICENSE.txt before you fork or resell.
Need a document wiki, not chat: Notion alternatives.
Need an LLM chat UI, not team chat: ChatGPT alternatives (Open WebUI).
Need workflow automation into Slack: Zapier alternatives.

What we left out

Discord-class community chat is now a separate hub: Discord alternatives (Element Matrix client; Rocket.Chat MIT-plus-EE). Revolt / Stoat stayed out of that pass (discussions-only redirect). Discord is a different product class (community / voice) than Slack (workplace).

Zulip · Mattermost
Writing & Office
Notion alternatives · ChatGPT alternatives
Zapier alternatives
Community / Matrix chat: Discord alternatives

Listed options

Zulip

Apache-2.0 topic-threaded team chat you can self-host or run on Zulip Cloud.

Apache-2.0

Mattermost

Open-core self-hosted team collaboration. Mixed LICENSE.txt (compiled MIT, source AGPL).

other

FAQ

What is the best free Slack alternative I can self-host?

Among options we sourced on 2026-10-23, Zulip Server 12.3 is Apache-2.0 (26,095 stars) with topic-based threading. Mattermost v12.0.1 is the higher-star Slack-shaped option (39,410 stars) but LICENSE.txt is mixed (compiled MIT, source AGPL, enterprise source-available). “Best” depends on license simplicity versus channel-shaped UX. We have no bake-off.

Is Mattermost fully open source?

Not as a single OSI SPDX. LICENSE.txt (2026-10-23) licenses compiled Mattermost, Inc. binaries as MIT, source you compile as AGPL v3.0 or commercial, some admin trees as Apache-2.0, and server/enterprise as source-available. GitHub’s license field is NOASSERTION. Zulip’s LICENSE is Apache-2.0.

Does Zulip work like Slack channels?

Zulip’s README describes streams plus topics, not Slack-style channels-only. That is the product difference we sourced, not a quality rank. Mattermost is the closer channel-shaped row on this hub.

Can I use these commercially?

Apache-2.0 (Zulip) allows commercial use of that code. Mattermost’s compiled MIT vs AGPL-source split is in LICENSE.txt; enterprise features need their E20 terms. Not legal advice.

Sources & verification

Last verified 2026-10-23 · Last updated 2026-10-23· Edited by forkalt editors · Editorial policy

License notes are not legal advice. Read the original LICENSE. Trademark & disclaimer.