I need a team chat app that is open source and does not claim ownership of our data.

Yes. If your key requirement is “our organization retains ownership/control of our chat data”, I’d focus on self-hosted open-source products rather than merely “privacy-friendly” hosted services.

My top picks

1. Zulip — best overall for data sovereignty

• Zulip explicitly says “You own your data” and supports self-hosting its 100% open-source software.
• Its cloud offering treats the customer as the data controller, but self-hosting gives you substantially more control.
• Its threaded/topic-based conversation model is particularly good for teams that have lots of ongoing discussions.
• My pick if data ownership is your #1 criterion.

2. Mattermost — best Slack-like option

• Mattermost Team Edition is open source under the MIT license.
• For self-hosted deployments, Mattermost says the customer controls the processing of end-user data.
• It's a more conventional channels/DMs/team-chat experience, so migration from Slack or Teams is relatively intuitive.
• Excellent choice if you want a familiar Slack-style interface.

3. Rocket.Chat — powerful, but read the licensing carefully

• Its self-hosted deployment says Rocket.Chat cannot access customer user-generated data.
• However, Rocket.Chat uses a mixture of open-source and proprietary code; its Community Edition is MIT-licensed while Enterprise Edition has additional restrictions.
• Its self-hosted terms also distinguish customer data from usage data, with Rocket.Chat retaining ownership of the latter.
• Good technically, but less clean if your requirement is specifically “the vendor shouldn't claim ownership of our data.”

What I'd choose

| Data sovereignty | Open source | Slack-like | My take --- | --- | --- | --- | --- Zulip | ⭐⭐⭐⭐⭐ | ⭐⭐⭐⭐⭐ | ⭐⭐⭐ | Best for your requirement Mattermost | ⭐⭐⭐⭐⭐ | ⭐⭐⭐⭐⭐ | ⭐⭐⭐⭐⭐ | Best familiar alternative Rocket.Chat | ⭐⭐⭐⭐ | ⭐⭐⭐⭐ | ⭐⭐⭐⭐⭐ | Powerful, but licensing deserves scrutiny

One important distinction: owning your content isn't the same as the vendor owning zero data about you. For example, Mattermost's self-hosted privacy policy says it may collect limited service/usage information unless the customer opts out.

If you tell me how many people are on your team and whether you want to self-host it on your own server, I can narrow this down to the best 2–3 options and compare data ownership, licensing, encryption, backups, mobile apps, SSO, and ease of migration from Slack/Teams.

Mattermost, Element (Matrix), Zulip, and Rocket.Chat are the top open-source team chat apps that give you 100% ownership of your data when you self-host them on your own infrastructure . By hosting these platforms yourself, no third-party vendor can claim ownership, read, or train AI models on your workspace data.

Top Open-Source Data-Sovereign Chat Options

• Mattermost : Best for developer and technical teams wanting a Slack-alternative interface. It features rich markdown, file sharing, and robust enterprise compliance, and can be deployed entirely on-premises or on a private cloud. Check out the Mattermost Self-Hosting Guide.
• Element (Matrix) : Best for maximum security and decentralized, end-to-end encrypted communication. Built on the open Matrix protocol, your messages federate or stay strictly within your own homeserver, ensuring total data privacy. Explore setup details via Element.
• Zulip : Best for async-first and structured conversations . Zulip uses a unique threading model where every message requires a topic, keeping large teams organized without information getting lost in a fast-moving stream. Learn more at Zulip Self-Hosting.
• Rocket.Chat : Best for organizations needing internal team communication paired with omnichannel customer engagement . It provides full data residency control and flexible self-hosted deployment. Review options on the Rocket.Chat Blog.

If you'd like to narrow this down, let me know:

• Do you need end-to-end encryption for all messages?
• Will you be hosting this on a local server or a private cloud VPS (like AWS/DigitalOcean)?
• Do you prefer Slack-style real-time channels or topic-threaded discussions?

When you want true data sovereignty—meaning no third-party claims or telemetry—the solution is to use self-hosted, open-source team chat software . By hosting the server on your own hardware or private cloud, 100% of the data stays in your possession.

The top open-source team chat options that offer complete data ownership include:

• Mattermost – The closest open-source equivalent to Slack. It is heavily optimized for engineering and enterprise teams, featuring robust markdown support, file sharing, and DevOps integrations. You can deploy it via Docker on your own server, and your data never leaves your infrastructure.
• Rocket.Chat – A feature-rich, highly customizable communication platform that serves as an open-source alternative to Slack and Microsoft Teams. It supports live chat, channels, direct messaging, and video conferencing while ensuring total data compliance (GDPR, HIPAA).
• Zulip – Known for its unique threading model where conversations are organized by both channels and topics. It is exceptional for keeping remote or asynchronous teams organized, and the entire server code is open source and ready for self-hosting.
• Element (Matrix) – Built on the decentralized Matrix protocol, Element provides end-to-end encrypted communication. You can run your own Matrix homeserver, giving you absolute control over your communication metadata and message storage.
• Nextcloud Talk – If you are already looking for a broader collaborative ecosystem (file storage, calendars, documents), Nextcloud's on-premise chat and video calling module integrates seamlessly while retaining complete local data custody.

To help narrow down the best fit, tell me:

• Do you prefer a Slack-like interface (Mattermost/Rocket.Chat) or a threaded/topic-based layout (Zulip)?
• Will you be self-hosting on your own hardware/VPS , or looking for a specific deployment environment?

If your priority is open source + self-hosting + the vendor explicitly not claiming ownership of your messages/files, I’d shortlist these:

My recommendation: Zulip

zulip.com is unusually explicit about this requirement. Its documentation says “You own your data” and that self-hosting gives you full data sovereignty. The server is 100% open source and distributed under the Apache 2.0 license.

It also has a useful topic-based conversation model, which can be considerably more organized than Slack-style channels once a team gets busy.

If you want something more like Slack

Rocket.Chat is probably the closest match. Its terms explicitly state that the customer retains intellectual-property rights in user-submitted content, and its self-hosted privacy policy says Rocket.Chat cannot access user-generated data on a self-hosted instance.

One caveat: Rocket.Chat has both open-source Community Edition and proprietary Enterprise components, so you need to be deliberate about which edition you deploy.

Another strong option: Mattermost

Mattermost Team Edition is open source under the MIT license, with self-hosting available. Mattermost Documentation GitHub It's particularly attractive if your organization cares about administrative controls, security, and keeping everything inside its own infrastructure.

Bottom line: If you tell me your priorities—e.g. Slack-like UI, mobile apps, video calls, SSO, end-to-end encryption, easy Docker deployment, or 10/50/500+ users—I can narrow this to the best 1–2 choices.

If your priority is open source + you keep ownership/control of your data, look for a platform you can self-host (run on your own server) and whose license does not give the vendor ownership of your messages/files.

Good options:

My shortlist based on your requirement:

1. Zulip — best fit if you want a truly open-source team chat with strong data control and a clean license.
2. Rocket.Chat — best fit if you want something that feels closest to Slack/Teams and has many integrations.
3. Element/Matrix — best fit if privacy, federation, and avoiding dependence on one vendor matter most.

Things I would verify before adopting any chat platform:

• Can you export all messages, files, and metadata?
• Can you run it without the vendor’s cloud?
• Is the server code under a true open-source license (not just “source available”)?
• Do you control encryption keys and backups?
• Does the vendor claim a license to use your content for their purposes?

If you want the closest open-source replacement for Slack, I’d start with Zulip or Rocket.Chat. If you want the strongest “our data stays ours” posture, choose self-hosting rather than a vendor-hosted plan.

If you want absolute data ownership, the key is to use an open-source platform that you self-host (run on your own hardware or private cloud server). When you host the server yourself, no third-party company can claim ownership, mine your data, or change their terms of service out from under you.

The top open-source team chat applications that provide complete data sovereignty include:

• Mattermost: Widely considered the best open-source, self-hosted alternative to Slack. It is fine-tuned for developer and enterprise collaboration, features robust markdown support, file sharing, and deep DevOps/secops integrations. It can be deployed easily via Docker on your own infrastructure.
• Rocket.Chat: A feature-rich [7], highly scalable open-source platform that supports team channels, direct messaging, guest access, and built-in video conferencing [7]. It’s heavily used by enterprises and government agencies needing strict data residency compliance and air-gapped deployments.
• Element (Matrix): Built on the decentralized Matrix protocol , Element offers secure, end-to-end encrypted team communication. Instead of a single centralized server, it operates on a federated network model where you can run your own home-server and control every byte of data your team exchanges.
• Zulip: A powerful open-source team chat that uses a unique threaded conversation model rather than traditional flat channels. It keeps large teams organized and is fully self-hostable with complete ownership of message histories and files.
• Nextcloud Talk: If you are already looking for a broader collaboration suite, Nextcloud is an open-source, self-hosted productivity ecosystem. Its Talk app provides secure on-premise text chat, audio, and video calls tightly integrated with file storage and calendars.

To help narrow down the best fit, let me know:

• Do you prefer a Slack-like interface (Mattermost/Rocket.Chat) or a threaded layout (Zulip)?
• Will you be hosting this on a local server/NAS or a cloud VPS (like DigitalOcean, AWS, etc.)?

If you want an open-source team chat app that guarantees absolute data ownership, you need a platform you can self-host on your own hardware or private cloud . When you host the server yourself, no third party can claim ownership, mine your messages for ad profiling, or lock your history behind a paywall.

The top open-source, self-hostable team chat platforms tailored for complete data sovereignty include:

• Mattermost: Best if you want a polished, direct alternative to Slack. It is heavily optimized for engineering, DevOps, and high-compliance teams. It features robust file sharing, markdown support, search, and extensive webhook integrations. Because you run it on your own server, every byte of data stays strictly under your control.
• Rocket.Chat: Best for an all-in-one communications hub. It mirrors Slack's feature set closely—including live chat, guest access, and video conferencing—making it ideal for organizations that want deep customizability and complete data sovereignty on-premises.
• Matrix (via Element): Best for maximum decentralization and end-to-end encryption. Matrix is an open standard for secure, decentralized communication, and Element is its flagship client. It operates similarly to email or the fediverse—you can host your own server and federate with others, meaning your data lives exclusively where you put it.
• Zulip: Best for async-first and highly technical teams. Instead of a traditional flat chat stream, Zulip organizes conversations by streams and topics , making it uniquely powerful for keeping complex, long-running technical discussions structured and searchable over time.

To help narrow down the right option, tell me:

• Do you plan to self-host on your own server/VPS , or are you looking for a hosted open-source provider?
• How large is your team, and do you need video/audio calling built-in?

Yes. If data ownership/control is a hard requirement, I’d focus on self-hosted open-source chat rather than hosted SaaS.

My top picks

1. Zulip — best overall

• 100% open source, including the server.
• Self-host it and retain control of your data.
• Excellent threaded/topic-based conversations, which is particularly good for teams with lots of ongoing discussions.
• Zulip explicitly says you control your data and emphasizes avoiding vendor lock-in.
• It also provides export/import tools so you can move your data elsewhere.

2. Rocket.Chat — best Slack-like alternative

• Community Edition is open source and MIT-licensed.
• With a self-managed installation, Rocket.Chat says it does not access your user-generated content.
• Supports messaging, group chats, file sharing, integrations, calls, and more.

3. Mattermost — good for technical/DevOps teams

• Team Edition is open source, self-hosted, and MIT-licensed.
• Particularly strong if your team wants ChatOps/developer tooling integrations.
• One caveat: Mattermost itself currently recommends Team Edition mainly for smaller teams and says it isn't intended for sensitive commercial workloads.

What I'd choose

Zulip would be my first choice if your priority is:

“We want a modern team chat system, but our organization—not the vendor—should control the data.” It's unusually explicit about this philosophy: its self-hosted version is 100% open source, gives you full control of the data, and is designed to avoid lock-in.

If you want something that feels closest to Slack, I'd pick Rocket.Chat. Its legal terms also explicitly state that Rocket.Chat does not obtain ownership of Customer Data.

Important distinction: “open source” by itself doesn't guarantee that a hosted provider can't process your data. For the strongest version of your requirement, self-host the server yourself (or have a trusted infrastructure provider host it under your control).

If you tell me roughly how many people are on the team and whether you need video calls, mobile apps, SSO, or Slack import, I can narrow this to the best 1–2 choices.

The top open-source team chat applications that allow you to completely self-host and retain 100% ownership of your data include Mattermost, Rocket.Chat , and Element (Matrix).

By self-hosting any of these platforms on your own hardware or private cloud infrastructure, you ensure that no third party claims ownership or harvests your organization's communication data.

If you'd like, I can:

Explain how to self-host Mattermost using Docker Compare Element and Mattermost in detail Provide hardware requirements for hosting a team chat server

If your priorities are open source + keeping ownership/control of your team's data, I’d shortlist these:

• Zulip — my top pick. Its self-hosted edition is explicitly described as 100% open-source and lets you retain full control of your data. Importantly, even Zulip's hosted terms state that your User Submissions remain yours and that the terms don't give Zulip ownership of them.
• Element / Matrix — best for maximum data sovereignty. Element specifically describes its model as keeping ownership and control of your data, with self-hosting/on-premises/private-cloud deployment. Matrix also makes it possible to change hosting providers without being locked into one vendor.
• Rocket.Chat — good Slack-like option. Its self-managed deployment states that Rocket.Chat does not store user-generated content, and its terms say the customer retains ownership of intellectual-property rights in user data. Be aware, however, that Rocket.Chat is a mixture of open-source and proprietary components; the Community Edition is MIT-licensed, while Enterprise Edition is source-available/commercial.
• Mattermost — another strong self-hosted choice. Its Team Edition is open source under the MIT license. I'd favor the self-hosted Team Edition if data control is the deciding factor rather than using a hosted service.

What I'd choose

For a normal company/team: Zulip. It's genuinely open source, mature, self-hostable, and its terms explicitly distinguish ownership of your content from ownership of the software.

For maximum sovereignty/privacy: Element + your own Matrix server. Your organization controls where the communications data lives rather than entrusting it to a chat vendor.

One important distinction: "open source" doesn't automatically mean "the vendor can't claim rights to your data." I'd look separately at the software license, the hosted-service terms, and whether you can self-host. If you self-host, the practical control is much stronger because the vendor doesn't possess your chat database in the first place.

If you tell me how many people are on the team and whether you need Slack-like channels, video calls, file sharing, or mobile apps, I can narrow this to the best 1–2 choices.