Slack vs Mattermost

Slack is a paid team chat service with a free plan [S1]. Mattermost is a team chat project with its source on GitHub under the GNU AGPL v3.0 or a commercial license [S2]. We checked every figure on this page against the vendors' own pages on 16 Oct 2026. Prices on this page are list prices as of that date.

Slack vs Mattermost at a glance

What is Mattermost?

Mattermost's code is on GitHub at mattermost/mattermost. Its LICENSE.txt offers the source "Under the Free Software Foundation’s GNU AGPL v3.0" [S2]. That is "subject to the exceptions outlined in this policy" [S2]. The other option is a commercial license [S2]. The GitHub API does not return a single SPDX id for it. Forkshelf records it as AGPL-3.0-only with an "Open core" note [S4]. The repo had 39,557 stars on 8 Nov 2026 [S3]. It is not archived, and the last push was on 8 Nov 2026 [S3].

Mattermost vs Slack pricing

Slack's Free plan costs US$0 and includes 90 days of message history [S1]. Slack Pro costs US$8.75 per user a month when you pay monthly [S1]. It costs US$7.25 per user a month when you pay annually [S1]. These are list prices. Slack is currently showing a "50% off for 3 months" promotion on its pricing page [S1].

Mattermost's source is available under AGPL v3.0 [S2]. Mattermost's own paid plans are not documented on the pages we read. In our reading, a self-hosted Mattermost costs you the server and the time to run it.

Slack is the better fit when

• Your team can work with 90 days of message history on a free plan [S1].
• You want a hosted plan with a listed per-user price [S1].

Mattermost is the better fit when

• You want the chat source under the GNU AGPL v3.0 [S2].
• You want a project with recent activity. The repo had a push on 8 Nov 2026 [S3].

Run Mattermost on Railway

Railway lists a community template by shinyduo for Mattermost (railway.com/deploy/mattermost) [S5]. Railway shows it as unverified [S5]. Railway verifies a template only when the maker of the technology "becomes a partner and reviews the template" [S7]. The template had 96 projects on 16 Oct 2026 [S5]. Read the template page and its source before you deploy.

The template is titled "Deploy & Host Mattermost [Updated Oct '26]" [S5]. It runs the image mattermost/mattermost-team-edition:latest [S5]. Volume needs are not stated on the template page.

Affiliate disclosure: Forkshelf earns a commission if you sign up for Railway through the links marked "Paid link" on this page. This does not change the order or content of any comparison here.

To get started, create a Railway account (Paid link). The Free plan allows up to 1 vCPU and 0.5 GB RAM per service, with 0.5 GB of volume storage per volume [S6]. The Hobby plan has a US$5 minimum usage per month, with US$5 of monthly usage credits included [S6][S8]. It allows up to 5 GB of volume storage per volume [S6].

Railway is one option. Pricing on Railway is as of 16 Oct 2026.

Other Slack alternatives

Forkshelf lists 45 tools with open-source alternatives. See the Slack alternatives page for the open-source options we list for Slack.

FAQ

Is Mattermost open source?
Its source is offered under the GNU AGPL v3.0, with listed exceptions, or under a commercial license [S2]. Forkshelf marks it as open core [S4].

How much is Slack Pro?
US$8.75 per user a month billed monthly, or US$7.25 billed annually, at list price as of 16 Oct 2026 [S1].

How long does Slack Free keep messages?
The Free plan includes 90 days of message history [S1].

Sources (all fetched 16 Oct 2026)

• [S1] https://slack.com/pricing
• [S2] https://github.com/mattermost/mattermost/blob/master/LICENSE.txt
• [S3] https://api.github.com/repos/mattermost/mattermost
• [S4] Forkshelf listing for mattermost/mattermost, https://forkshelf.com/alternative-to/slack
• [S5] https://railway.com/deploy/mattermost
• [S6] https://railway.com/pricing
• [S7] https://docs.railway.com/templates/publish-and-share
• [S8] https://docs.railway.com/pricing/plans