Open-source alternatives to Slack

Which open-source alternative to Slack fits your situation

If you need a self-hosted replacement for Slack that is closest in day-to-day feel and has a documented migration path from a Slack export, Mattermost is the most direct fit, with Rocket.Chat as the second option to evaluate on the same criteria. If your priority is federated, interoperable messaging across organizations and you are willing to run a Matrix homeserver, Matrix with Element is the right family to test. Most teams searching for a Slack alternative are reacting to one of four triggers: a renewal or price change, a licensing change, a data residency requirement, or lock-in to a platform whose history they cannot fully control. The pressure usually starts with retention. Slack's Free plan keeps 90 days of message and file history, and Slack states that content on Free workspaces older than one year is permanently deleted, so an organization on that plan may already be losing records it expects to keep. Exporting is also tiered: every plan can export messages and file links from public channels, but exporting private channels and direct messages requires Business+ or Enterprise, and custom exports by member or conversation are Enterprise-only. Before you pick a destination, confirm with Slack which plan your workspace is on, which export scope your records team needs, and whether private channels and DMs are in scope for retention or legal hold. Those answers determine how much history you can extract at all. The decision then turns on identity, licensing and sovereignty. Mattermost Team Edition is free, self-hosted and aimed at teams under 250 users that do not need single sign-on, so larger or regulated deployments should plan for a paid edition that adds SSO, AD/LDAP sync and compliance export. Rocket.Chat keeps its code outside enterprise directories under MIT, and its Commercial, Government and Defense plans cover on-premises and air-gapped deployments. Matrix with Element fits when federation matters most, and Element's Server Suite includes a free AGPL Community edition, a Pro edition and a Sovereign edition for air-gapped use. Confirm license terms with each vendor, because AGPL obligations apply to modified network-facing code.

Mattermost, Rocket.Chat and Matrix with Element as self-hosted Slack replacements

Each of these platforms can run inside your own cloud account, data center or air-gapped network, which removes the vendor from the message store. They differ in licensing, in how much of the enterprise feature set is free, and in how well they fit federated or air-gapped requirements. Compare them against your user count, single sign-on needs and deployment model before you choose.

Mattermost Team Edition

Mattermost Team Edition is free, self-hosted and MIT-licensed as compiled by Mattermost, and it targets teams under 250 users that do not need single sign-on. It is the closest match for a small Slack workspace that wants channels, direct messages and threads on its own servers. The trade-off is the ceiling: SSO, AD/LDAP sync, clustering and compliance export sit in paid tiers, so confirm your identity and audit requirements before you commit. Check the user count and SSO need first.

Mattermost licensing and paid tiers

Mattermost source code is available under AGPL v3.0 or under a commercial license from Mattermost, Inc., and admin tools and configuration files are under Apache 2.0. The free Entry tier of the commercial build caps channel message history at 10,000 messages and has no compliance features or high availability. Ask your legal team which license applies to your planned use, and whether an AGPL obligation affects any modifications you make. Read the license split before you fork or customize.

Rocket.Chat Starter and paid plans

Rocket.Chat offers a free self-managed Starter workspace for evaluation and small teams, with Commercial, Government and Defense plans for on-premises, air-gapped and sovereign deployments. Code outside its enterprise directories is MIT-licensed, while code under the ee directories uses Rocket.Chat's enterprise license. Map each feature you need to the correct tier, because the free workspace and the paid editions are not the same product. Confirm which features are in the ee directories.

Matrix with Element Server Suite

Matrix is an open federation protocol, and Element Server Suite runs Synapse, its Matrix homeserver, with Element clients on top. Synapse and Dendrite moved from Apache 2.0 to AGPLv3 on November 6, 2023, with commercial licenses available from Element. The Community edition is free under AGPL and includes Synapse and Element Call, while the Sovereign edition targets air-gapped deployments. Federation and end-to-end encryption suit cross-organization chat, but expect more operational work than a single-server product. Verify the edition's feature set against your audit needs.

Air-gapped and sovereign deployment

Air-gapped operation is where the three options diverge most, and it should decide the shortlist for regulated networks. Rocket.Chat and Element both offer paid editions aimed at air-gapped or sovereign environments, while Mattermost's compliance and high-availability features sit in paid tiers. Ask each vendor for written confirmation of offline installation, update and license validation steps. Confirm offline update paths before you rely on any option.

Slack data migration fit

Mattermost and Rocket.Chat both document import paths from a Slack export, but they carry different content. Mattermost moves users, channels, direct messages, threads and attachments, while integrations, custom emoji images, user groups and Slack Connect channels do not transfer. Rocket.Chat imports users, channels and messages from a Slack export through its admin Import screen. Export private channels and direct messages before your Slack plan changes, because that export needs Business+ or Enterprise. Plan a gap list for integrations and Slack Connect channels.

Which Slack capabilities each open-source option covers

This table maps the Slack capabilities most teams depend on to Mattermost, Rocket.Chat and Matrix with Element. Read each cell as a starting point: where the supplied facts are silent, confirm the detail with the vendor or in a test deployment before you commit.

Capability	Mattermost	Rocket.Chat	Matrix with Element
Channels and direct messages	Public and private channels and direct messages, with Slack import support for these objects	Channels and direct messages, with Slack import support for these objects	Rooms and direct messages, with the Element client; confirm room and admin controls for your edition
Threads and file sharing	Threads and attachments migrate from Slack; confirm file size limits for your deployment	Threads are available; confirm file storage options and size limits for your deployment	File and thread support depends on the client and server build; confirm in a pilot
Search and message history	Entry edition caps channel message history at 10,000 messages; confirm history limits for your tier	Confirm history and search behavior in your edition and plan	Confirm search scope and history retention on your Synapse or Element Server Suite edition
SSO and AD/LDAP sync	Paid Professional, Enterprise and Enterprise Advanced tiers; not in the free Team Edition or Entry	Confirm SSO and directory sync in the plan that matches your deployment	Confirm identity integration in the Element Server Suite edition you select
Compliance export and retention	Compliance export is in paid tiers; Entry has no compliance features	Confirm export and retention controls in the Commercial or Government plan	Confirm export and retention tooling with Element for your edition
Integrations and apps	Slack integrations do not migrate; rebuild or replace them and confirm webhook and bot compatibility	Slack integrations do not migrate; confirm app and webhook support for your needs	Bridges and bots vary by ecosystem; confirm each integration you need
Slack Connect channels	Slack Connect channels do not migrate; plan a separate cutover for shared external channels	Confirm handling of shared external channels; treat as a migration gap until verified	Confirm bridging options for externally shared rooms before relying on them
Air-gapped or sovereign deployment	Self-hosted in your own infrastructure; confirm the edition that fits your requirements	Commercial, Government and Defense plans support on-premises, air-gapped and sovereign deployments	Element Server Suite Sovereign edition is offered for air-gapped deployments

What you give up when you leave Slack and the usual workarounds

Moving off Slack means accepting that some things will not come with the data. Integrations, custom emoji images, user groups, Slack Connect channels and hosted availability each need a named owner and a decision before cutover.

Integrations and apps do not migrate

Slack apps, bots, slash commands and incoming webhooks are not carried over in a Slack import. Mattermost's migration path documents that integrations do not move, and Rocket.Chat's import covers users, channels and messages. Inventory every app and webhook in your workspace first, then decide whether each one is rebuilt against the target platform's API, replaced by a built-in feature, or retired. Confirm webhook endpoint formats with the vendor before you repoint any production system.

Custom emoji images stay behind

Mattermost's Slack migration does not move custom emoji images, so custom reactions can disappear from message history. Plan to export the image files from your Slack admin pages, then upload them to the target server with the same shortcodes. Check whether your Rocket.Chat import preserves custom emoji on your version, since this is not covered by the sources used for this page. Keep a list of the emoji that teams rely on in daily work so you can prioritize them.

User groups must be recreated

User groups, such as handles used for mentions in Slack, are not part of the Mattermost Slack migration. Recreate them as teams, channels or groups on the target platform. If your identity provider already manages membership, use directory sync to drive group membership instead of maintaining it by hand. Confirm which group features and directory sync options your edition includes, because some are limited to paid tiers.

Slack Connect channels do not carry over

Slack Connect channels shared with partners, vendors or customers are not migrated, and the other organization's members will not follow you. Before cutover, list each shared channel and the external contact who owns it. Options include federation where the target supports it, such as Matrix bridges for specific external networks, or a planned invitation of external users to a new shared space. Confirm with each partner whether their own security policy allows an external federated or self-hosted server.

Hosted availability becomes your duty

Slack runs as a hosted service, and your team takes on uptime, patching, backups and restore testing when you self-host. Define recovery objectives, a backup schedule and an upgrade cadence before go-live. Mattermost reserves clustering and high availability for paid tiers, so confirm whether your deployment needs them. Ask your DeployOpen or internal platform team to write the runbook and test a restore before retiring Slack.

App marketplace and vendor ecosystem

Slack's app directory has no direct equivalent on self-hosted platforms. Check whether each business-critical app has an open API, a webhook or a maintained connector for the target platform, and treat anything without one as a build item. Ask each vendor whether it supports self-hosted chat before you commit, because some offer only hosted integrations. Record the outcome in a single decision log so procurement and security sign off on the same list.

How to move a Slack workspace to Mattermost or Rocket.Chat

This sequence moves channels, users and message history from Slack into a self-hosted workspace, with a pilot and parallel run so the team can switch over with a confirmed record of what moved.

Inventory channels, members and retention needs
The workspace owner and a compliance contact list every public and private channel, direct message group, active member, installed integration and Slack Connect channel. Record which conversations must be retained and for how long, since Slack's Free plan keeps 90 days of history and permanently deletes content older than one year. Note that integrations, custom emoji images, user groups and Slack Connect channels do not transfer to Mattermost, so decide in advance how each will be rebuilt or retired. The step is done when the team has a signed-off inventory and a retention decision for each category.
Export the Slack workspace data
A Slack workspace owner or admin requests the export from the workspace settings. Every plan can export messages and file links from public channels, but exporting private channels and direct messages requires a Business+ or Enterprise plan, and custom exports by member or conversation are limited to Enterprise. If the organization is on a lower plan, confirm the upgrade before starting, because private history cannot be recovered from a plan that does not export it. The step is done when the export archive is downloaded, checked for the expected channels and stored in an access-controlled location.
Convert and load the export into the target platform
For Mattermost, an engineer converts the Slack export with mmetl into Mattermost's bulk import format, then loads it with mmctl against a staging server. For Rocket.Chat, an administrator opens Manage, Workspace, Import and uploads the Slack export, after splitting large archives into files under 15 MB as the Rocket.Chat documentation recommends. Confirm that users, channels, direct messages, threads and attachments appear as expected. The step is done when a sample of channels and conversations has been checked against the original Slack archive and any import errors are logged and resolved.
Map users and configure authentication
The identity owner matches each Slack account to the corresponding directory identity, resolves duplicates and removes former employees or guests who should not receive access. Decide how users sign in, whether through local accounts or an identity provider, and confirm that the chosen edition supports that method, because single sign-on and directory sync sit in paid tiers for Mattermost. The step is done when a mapping file has been reviewed and every active user can sign in to the staging environment with the intended method.
Run a pilot with a defined group
A small group of representative users, including one or two team leads and a compliance reviewer, works in the new platform for a set period agreed by the project owner. Testers check message search, file access, notifications, mobile clients, bots that replace Slack integrations and permissions on private channels. Each issue is logged with an owner and a fix or accepted limitation. The step is done when the pilot group signs off on the required workflows and the open issues list contains only items the business has accepted.
Run both platforms in parallel, then cut over
Announce the cutover date, keep Slack available read-only for the agreed overlap period, and redirect new work to the self-hosted platform. Teams confirm that critical notifications, on-call routing and external collaboration paths have moved, and any Slack Connect partners receive a new channel invitation. Operations watches capacity, backups and logs during the first days of live use. The step is done when all active users are on the new platform and the cutover runbook has recorded the sign-off from the business owner.
Decommission the Slack workspace
Once the retention period has ended and the archive is verified, the workspace owner removes integrations, revokes app tokens and disables remaining accounts. The compliance team confirms that the archived export and migration logs are stored according to the retention schedule, then closes the Slack subscription. Confirm the exact handling of deleted or inaccessible content with Slack before closing the account. The step is done when the subscription is cancelled, the archive location and retention owner are documented and the access review has been filed.

Which Slack alternative fits data residency, cost, customization or regulated use

When the main requirement is that messages, files and metadata stay in a specific country, cloud account or data center, all three options can run self-hosted, so the choice comes down to operational fit. Mattermost and Rocket.Chat both store their data in a database and file store that your team controls, which makes residency a matter of where you deploy them. Matrix with Element is a federated protocol, so residency also depends on which homeserver your users join and whether you federate with outside servers.

Mattermost Team Edition: Suits a single-region deployment for smaller groups that do not need single sign-on.
Rocket.Chat self-managed: The free Starter workspace is for evaluation and small teams, while Commercial, Government and Defense plans support on-premises, air-gapped and sovereign deployments.
Element Server Suite Sovereign edition: Built for air-gapped deployments, which matters when residency rules also restrict outbound network paths.
Matrix federation: Turn federation off or restrict it to approved servers if messages must not leave your jurisdiction.
Condition where it fails: If your team needs managed hosting in a region the vendor does not operate, self-hosting moves the residency burden to your own infrastructure.

Before choosing, confirm with each vendor which deployment models their current plans cover and map every backup, log and search index location to your residency rules.

Questions buyers ask before replacing Slack

How much effort does replacing Slack need from our own team?

Replacing Slack is mostly an operations and change project, and the largest effort falls on your own staff: someone must own the server, identity integration, retention policy and user adoption. Plan for named owners in infrastructure, security and the business teams that depend on channels. Ask any implementation partner for a written scope listing which tasks they perform and which stay with your team, since that split drives the internal hours you need to commit.

How long does a Slack migration take, and what drives the timeline?

No supplied figure sets a fixed duration, so treat any timeline as something to confirm in writing. The main drivers are the size of the Slack export, how many channels and direct messages you keep, how many integrations must be rebuilt, whether single sign-on and directory sync are required, and how many approval gates your security review contains. Mattermost's import path depends on a Slack export converted with mmetl, and Rocket.Chat asks you to split large exports into files under 15 MB, so export volume directly affects how many import runs you schedule.

Can our team migrate from Slack ourselves?

Yes, a capable internal team can run the documented import paths, but the work goes beyond the import itself. Mattermost migrates users, channels, direct messages, threads and attachments through mmetl and mmctl, while integrations, custom emoji images, user groups and Slack Connect channels do not move. Rocket.Chat imports users, channels and messages through Manage, Workspace, Import. Your team still has to rebuild integrations, set up identity, harden the server and test restores, so compare your internal capacity against that list before deciding.

What Slack message history can we keep when we leave?

Every Slack plan can export messages and file links from public channels. Exporting private channels and direct messages requires the Business+ or Enterprise plan, and custom exports by member or conversation are limited to Enterprise. On the Free plan, Slack keeps 90 days of message and file history, and content older than one year on Free workspaces is permanently deleted. Confirm your current plan and export options with Slack before you cancel, because history you did not export may not be recoverable.

How do the AGPL, MIT and enterprise licenses differ for these platforms?

The license terms depend on which code you run. Mattermost source code is under AGPL v3.0 or a commercial license from Mattermost, Inc., with admin tools and configuration files under Apache 2.0, and Team Edition is described as MIT-licensed. Rocket.Chat code outside its enterprise directories is MIT-licensed, while code under the ee directories uses its enterprise license. Element relicensed Synapse and Dendrite from Apache 2.0 to AGPLv3 on November 6, 2023, with commercial licenses available. Have legal counsel review whether AGPL obligations apply to your modifications and internal use.

What should we request in a quote for a Slack replacement?

Ask for a written, fixed scope that names each platform, the edition and features included, and the deliverables. Request line items for discovery, identity integration, server build and hardening, the migration runs from your Slack export, integration rebuilds, user training, backup and restore testing, and handover documentation. Ask which items are one-time and which are ongoing retainers for upgrades and support. Confirm what the quote excludes, such as commercial licenses for enterprise features, and ask which fees are paid to the platform vendor rather than to the implementer. DeployOpen does not publish a price list, and a quote follows from the written scope.

Does DeployOpen certify our Slack replacement as compliant?

No. DeployOpen prepares systems, controls and evidence for compliance assessments, but independent auditors and certification bodies issue audit reports, attestations and certifications. Ask for the evidence package in the scope, such as configuration records, retention settings, access control documentation and backup test results, so your assessor has what it needs. Confirm with your auditor which evidence they expect before the work starts.

What moving off Slack involves for your team

The import is the smallest part of the move. Once the export has been converted and loaded, your internal team owns identity, retention, integrations, adoption and the cutover decision, and none of those are handled by the import tooling. Start by deciding how people will sign in to the new server. Mattermost Team Edition is aimed at teams under 250 users that do not need single sign-on, so if your directory is the source of truth for access, confirm early whether your target edition provides SSO and directory sync. Rocket.Chat and Element also differ on which identity features sit in free versus paid builds, so check each vendor's edition documentation rather than assuming parity. Map who gets accounts, who is an admin, and how deprovisioning on leavers will work, because imported users are not a substitute for an access model.

Retention is a policy decision your legal and compliance owners must make, not a setting you can leave at default. Slack's Free plan keeps 90 days of history and deletes content older than one year, so many teams arrive with a history they never intentionally kept. Exporting private channels and direct messages from Slack requires Business+ or Enterprise, and your export plan should be confirmed against the content you are obligated to keep. Mattermost Entry caps channel history at 10,000 messages and omits compliance export, so if you need retention or legal hold, you will need a paid build or a documented alternative. Write down the retention periods per content type before the import runs, and confirm with your records and legal teams which Slack content they need to see.

Integrations, change management and the parallel-run decision need named owners. The migration tooling does not move integrations, custom emoji images, user groups or Slack Connect channels, so inventory every bot, webhook and app your teams rely on and assign each one a rebuild or retirement decision. Plan training around the actual differences in navigation, notifications and mobile apps, and name champions in each business unit. Finally, decide whether Slack and the new server run in parallel, and for how long. A parallel run lowers the risk of losing a conversation during cutover but doubles notification noise and makes it unclear where decisions are recorded. Set the cutover criteria in writing, including who signs off, and confirm the duration of any overlap with your Slack contract terms.

How DeployOpen scopes, migrates and supports a Slack replacement

DeployOpen plans, implements, integrates, hardens, backs up, documents and supports self-hosted open-source chat platforms inside your own cloud accounts, data centers or air-gapped networks. Engagements are fixed-scope, with scope, deliverables and timelines agreed in writing before work starts.

How we scope

Inventory which Slack workspaces, channels, direct messages and integrations the replacement must cover
Decide between Mattermost, Rocket.Chat and Matrix with Element based on your identity, hosting and retention requirements
Confirm which Slack features, such as Slack Connect channels and custom emoji, will not carry over
Agree the written scope, deliverables and timeline before any build work starts

How we build

Deploy the chosen platform in your environment with the identity provider and network controls you specify
Run a test migration from a Slack export, reviewing users, channels, threads and attachments before cutover
Test the platform's security configuration, including access controls, and document the findings
Configure backups and retention settings to match the policies your compliance team has approved

How we hand over

Document the architecture, configuration, runbooks and the known gaps against the original Slack setup
Provide user and administrator guidance for the cutover period and the new platform
Walk your operations team through backup, restore and upgrade procedures before handover
Deliver compliance readiness evidence for your own auditors, who issue any attestation or certification

How we support

Ongoing support, upgrades and continuous testing are available as a retainer after handover
Plan platform upgrades, such as the releases your chosen vendor publishes, and test them before rollout
Confirm with your vendor which edition, license and support tier your deployment requires before you commit
Agree retainer scope, response expectations and reporting in writing with the quote