What is the best open-source alternative to Slack?

Q: What is the best open-source alternative to Slack?

Mattermost and Rocket.Chat are the top self-hosted Slack alternatives with full team messaging, channels, and integrations — at a fraction of the cost.

The two strongest production-ready open-source Slack alternatives are Mattermost and Rocket.Chat.

Mattermost

Mattermost is a team messaging platform built for organizations that need data sovereignty. It's used by governments, banks, and security-conscious companies who can't put communications on a third-party server.

Self-hosted: Deploy on your own infra, keep data inside your network
Cost: Free for the Community edition; ~$10/user/mo for Enterprise features
Channels, DMs, threads, search: Full parity with Slack
Integrations: Webhooks, bots, plugins — compatible with most Slack workflows

Rocket.Chat

Rocket.Chat is more consumer-friendly and supports video calling out of the box via Jitsi. Better if you need guest access or customer-facing channels.

Self-hosted or cloud: Flexible deployment
Cost: Free self-hosted; Pro plans from $7/user/mo
Omnichannel: Built-in support for email, LiveChat, and WhatsApp routing

Cost comparison (10-person team)

Tool	Monthly cost
Slack Pro	$87.50
Mattermost (self-hosted)	~$20 (infra only)
Rocket.Chat (self-hosted)	~$20 (infra only)

Migration effort

Mattermost has a Slack import tool — channels, users, and message history transfer in about 30 minutes. Rocket.Chat requires a bit more manual setup but has detailed migration guides.

Mattermost

Get the full migration guide