Slack defined modern team communication and remains the default choice for most companies — its polish, integrations ecosystem (2,400+ apps), and ubiquity make it the path of least resistance. But Slack's pricing model ($8.75/user/month for Pro) adds up fast for growing teams, and some organizations have legitimate concerns about sensitive communications living on Slack's infrastructure. Mattermost is the open source alternative that gives you Slack-like functionality with full control over your data and deployment.

Mattermost's interface will feel immediately familiar to any Slack user — channels, direct messages, threads, reactions, file sharing, and search all work as expected. Where Mattermost differentiates is in enterprise features that Slack gates behind expensive plans: unlimited message history on all tiers, self-hosted deployment options, custom data retention policies, and compliance features like eDiscovery and custom TLS certificates. For organizations in regulated industries (healthcare, finance, government), Mattermost's self-hosted model simplifies compliance significantly.

Choose Slack if your team values maximum integration breadth, the most polished mobile experience, and the convenience of a fully managed service. Choose Mattermost if you need to self-host for compliance or data sovereignty reasons, want to avoid escalating per-seat costs, or need deep customization like custom plugins, webhooks, and integration with internal tools that Slack's app directory doesn't cover.