How to Migrate from Slack to Mattermost: Complete Guide

Slack's pricing ($8.75-12.50/user/month) adds up fast. A 50-person team pays $5,250-7,500/year. Mattermost gives you the same team chat experience, self-hosted, for the cost of a VPS. Here's exactly how to migrate.

What You'll Need

• A server for Mattermost (4 GB RAM minimum, 2+ cores)
• Slack workspace admin access
• 2-4 hours for setup, 1-2 weeks for team transition

Step 1: Deploy Mattermost

# Docker Compose — fastest path
mkdir mattermost && cd mattermost
curl -o docker-compose.yml https://raw.githubusercontent.com/mattermost/docker/main/docker-compose.yml
curl -o .env https://raw.githubusercontent.com/mattermost/docker/main/env.example

# Edit .env with your domain
nano .env
# Set DOMAIN=chat.yourdomain.com
# Set MM_SQLSETTINGS_DATASOURCE

docker compose up -d

Configure your reverse proxy (Nginx/Caddy) to point your domain to Mattermost.

Step 2: Export Slack Data

1. Go to Slack Admin → Settings & Permissions → Workspace Settings
2. Click Import/Export Data → Export
3. Choose Standard Export (public channels) or request a Corporate Export (includes DMs, private channels)
4. Wait for the export to complete → download the ZIP file

Note: Free and Pro plans only export public channels. Business+ and Enterprise Grid can export everything.

Step 3: Import into Mattermost

# Use the Mattermost bulk import tool
mmctl import upload /path/to/slack-export.zip
mmctl import list available
mmctl import process <import-file-name>

Or via System Console → Import → Slack Import → Upload the ZIP.

What transfers:

• ✅ Public channel messages and history
• ✅ Channel names and descriptions
• ✅ User accounts (as deactivated — they'll need to set passwords)
• ✅ File attachments (within Slack export limits)
• ⚠️ Threads (imported as regular messages)
• ❌ Reactions and emojis (custom emojis need re-upload)
• ❌ Apps and integrations (rebuild separately)
• ❌ DMs (unless Corporate Export)

Step 4: Set Up Integrations

Replace Slack integrations with Mattermost equivalents:

Slack Feature	Mattermost Equivalent
Slack bots	Mattermost bots + webhooks
Slack apps	Incoming/outgoing webhooks + plugins
/slash commands	Custom slash commands
Slack Connect	Federated channels (Enterprise)
Huddles	Calls plugin (built-in)
Workflow Builder	Playbooks
GitHub notifications	GitHub plugin
Jira integration	Jira plugin
Google Calendar	Calendar plugin

Set up incoming webhooks:

# System Console → Integrations → Incoming Webhooks → Add
# Then use the webhook URL in your services:
curl -X POST https://chat.yourdomain.com/hooks/YOUR_HOOK_ID \
  -d '{"text": "Deployment complete ✅"}'

Step 5: Configure SSO (Optional)

// config.json — GitLab OAuth example
{
  "GitLabSettings": {
    "Enable": true,
    "Secret": "your-secret",
    "Id": "your-client-id",
    "Scope": "read_user",
    "AuthEndpoint": "https://gitlab.com/oauth/authorize",
    "TokenEndpoint": "https://gitlab.com/oauth/token",
    "UserAPIEndpoint": "https://gitlab.com/api/v4/user"
  }
}

Mattermost supports OIDC, SAML, LDAP, Google, GitLab, and Office 365 SSO.

Step 6: Onboard Your Team

1. Send invitations — Generate invite links or send email invitations
2. Set passwords — Imported users need to create new passwords
3. Install apps — Desktop (Windows, Mac, Linux) and mobile (iOS, Android)
4. Pin a getting-started message — Key differences from Slack
5. Run both platforms — Keep Slack active for 2-4 weeks during transition

Key Differences to Communicate

Slack	Mattermost
Channels sidebar	Channels sidebar (same!)
Threads	Threads (same!)
Huddles	Calls (built-in plugin)
Slack Connect	N/A (or federated)
/giphy	/giphy (plugin)
Custom emoji	Custom emoji (re-upload needed)

Cost Savings

Team Size	Slack Pro (annual)	Mattermost Self-Hosted	Savings
10 users	$1,050/year	$120/year (VPS)	$930/year
25 users	$2,625/year	$240/year (VPS)	$2,385/year
50 users	$5,250/year	$360/year (VPS)	$4,890/year
100 users	$10,500/year	$600/year (VPS)	$9,900/year

Common Issues

"Messages are out of order after import" — Slack exports include message timestamps. Mattermost preserves them during import. If ordering looks off, check timezone settings.

"File attachments are missing" — Slack's standard export has file size limits. Large files may not be included. Re-share them in Mattermost.

"Users can't log in" — Imported users are deactivated by default. Activate them in System Console → Users and have them reset passwords.

"Custom emojis are missing" — Export custom emojis from Slack separately and re-upload via Mattermost's custom emoji settings.

Timeline

Week	Task
Week 1	Deploy Mattermost, import data, set up integrations
Week 2	Invite team, run both platforms in parallel
Week 3	Move primary communication to Mattermost
Week 4	Sunset Slack, cancel subscription

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