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

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.

Yes. If “does not claim ownership of our data” is a hard requirement, I’d focus on self-hosted, fully open-source software rather than merely a privacy-friendly hosted service.

My top pick: Zulip

Zulip is probably the closest match to what you described:

• 100% open source, rather than an open-core product.
• You can self-host it on your own infrastructure, keeping control of the database and files. Zulip explicitly describes this as retaining full control over your data.
• Zulip's terms explicitly say “Your User Submissions remain yours” and that the terms do not give Zulip ownership of them.
• It has good export/import capabilities, so you're not locked into Zulip.
• Its topic-based organization is particularly good if your team has lots of ongoing discussions rather than just rapid-fire chat.

Also worth considering: Rocket.Chat

Rocket.Chat is another strong option if you want something that feels more like Slack/Discord.

For a self-hosted deployment, Rocket.Chat states that it cannot access customer user-generated data, and its terms say customers retain ownership of intellectual-property rights in their user data.

The important caveat is that Rocket.Chat isn't completely open source across all editions: its Community Edition is MIT-licensed, while Enterprise Edition contains proprietary components.

My ranking for your requirement

1. Zulip — best fit if data sovereignty and genuinely open-source software are priorities.
2. Rocket.Chat Community Edition — better fit if you want a more conventional Slack-like experience, but pay attention to which edition you're deploying.
3. Matrix-based chat (e.g. Element + a self-hosted Matrix server) — excellent if you want an open protocol and federation, but it's a somewhat different architecture from a conventional team-chat app.

One distinction I'd make: “the vendor doesn't own our data” isn't the same as “the vendor can't access our data.” If you self-host, you can achieve both much more cleanly. For your stated requirement, I'd choose self-hosted Zulip unless you specifically want Slack-style channels and UX.

If you tell me your team size and whether you need SSO, video calls, file sharing, or mobile apps, I can narrow it down to the best 2–3 options.

The best open-source team chat apps that give you full data ownership through self-hosting include Mattermost, Element (Matrix), Rocket.Chat, and Zulip . By hosting these on your own servers, your organization retains 100% control and ownership over all messages and files.

Top Open-Source Team Chat Apps

• Mattermost : A powerful, Slack-style communication platform designed for developer and enterprise teams. It supports extensive integrations, high security, and complete control when self-hosted on your own infrastructure.
• Element : Built on the decentralized Matrix protocol , Element allows secure, encrypted real-time communication. It supports federation, meaning different servers can securely communicate while keeping data fully sovereign.
• Rocket.Chat : A feature-rich, customizable workspace chat that fits regulated environments. It offers complete data privacy, Omni-channel capabilities, and easy self-hosting via Docker or cloud appliances.
• Zulip : A unique chat app that organizes conversations by threads rather than flat channels. It is fully open-source and self-hostable, making it great for keeping complex technical discussions organized.

Comparison of Key Features

Would you prefer a tool that is easy to set up on a managed cloud or one that you can self-host entirely on your own private server?

Yes. If your requirement is open source + self-hostable + the vendor does not claim ownership of your chat data, I’d shortlist these:

My pick: Mattermost

Mattermost is particularly explicit about this. Its self-hosted documentation says the customer controls processing of end-user data, and its Team Edition is open-source and MIT-licensed.

More importantly, Mattermost explicitly describes its goal as total data ownership: a third party shouldn't be able to prevent you from accessing your data, read/analyze/monetize it, or retain it after you end the relationship.

There is also a useful distinction: self-host it yourself rather than using a vendor-hosted cloud service. With self-hosting, your database, files, backups, encryption keys, and infrastructure remain under your control. Mattermost says its self-hosted system does not include message contents in its telemetry.

Rocket.Chat is another strong choice

Rocket.Chat's privacy documentation is unusually direct: for a self-hosted deployment, Rocket.Chat says it cannot access customer user-generated data. Its Community Edition is MIT-licensed, although its broader product contains proprietary Enterprise components.

Zulip if conversation organization matters

Zulip is 100% open source, rather than an "open-core" product, and explicitly says self-hosting gives you full control of your data. It also provides export/import tools intended to avoid vendor lock-in.

One important caveat

"Doesn't claim ownership of our data" and "vendor can never see our data" aren't quite the same thing. Self-hosting is the key requirement if you want the strongest possible control. Also review backups, push notifications, analytics/telemetry, marketplace apps, and integrations—those can create separate data flows even when the core application is open source.

If I were choosing for a normal business team: Mattermost first, Rocket.Chat second, Zulip third.
If privacy/decentralization is the overriding concern: Element/Matrix deserves a very close look.

To truly own your data and ensure no third party claims ownership or harvests your conversations, you need a solution that is both open source and self-hosted on your own hardware or private cloud infrastructure.

The top self-hosted, open-source team chat options vary depending on your team's workflow:

• Mattermost stands out as the best overall Slack alternative for technical and DevOps-heavy teams. It features deep integrations with tools like GitHub, GitLab, and Jira, supports true air-gapped deployments (zero internet dependency), and offers built-in incident playbooks. Its core code is accessible via the Mattermost GitHub Repository.
• Rocket.Chat is ideal if you want a feature-rich, highly customizable platform that feels immediately familiar to Slack or Microsoft Teams users. It handles standard messaging brilliantly and can even scale to omnichannel support if you ever need to tie in customer service live-chats. You can inspect the source on the Rocket.Chat GitHub Repository.
• Zulip is the top choice for asynchronous or distributed teams. Instead of a single chaotic message stream, Zulip uses a unique topic-threaded model (combining the feel of chat with the organized structure of email) so discussions never get lost. The self-hosted version retains full feature parity, and details are available via Zulip Server Documentation.
• Matrix (via Element) is built for maximum privacy and decentralization. Rather than a single standalone server, Matrix uses an open protocol where your team runs its own "homeserver," and Element acts as the polished, end-to-end encrypted client app. You can review the ecosystem at Matrix.org.

To help narrow down which app fits your setup, let me know:

• Roughly how many users need to connect?
• Do you prefer a standard channel structure (like Slack) or a topic-threaded layout (like Zulip)?
• Do you have a server or VPS ready to host it?

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

🥇 Rocket.Chat

Rocket.Chat is probably the closest match to what you're asking for.

With a self-hosted deployment, Rocket.Chat explicitly says it cannot access user-generated data, and that the administrator/organization controls the data. Its customer terms also say Rocket.Chat does not obtain ownership of Customer Data.

One important caveat: Rocket.Chat has both open-source and proprietary components/editions. The Community Edition is MIT-licensed, while Enterprise Edition has additional restrictions.

🥈 Zulip

Zulip is particularly good if you want conversations organized by topics within streams, rather than Slack-style channels that can become noisy.

Zulip's terms explicitly state that your User Submissions remain yours and that its terms don't give Zulip ownership of them. It also supports exporting an organization's data to a self-hosted Zulip server.

It's also genuinely open source—the project describes itself as 100% open source.

🥉 Mattermost

Mattermost is another mature Slack alternative and has an open-source Team Edition.

However, I'd put it behind Rocket.Chat and Zulip for your specific requirement because Mattermost's licensing and commercial offerings are more complicated, and its legal terms contain fairly broad provisions concerning its intellectual property and feedback.

My recommendation: If you're comfortable running the server yourself, I'd choose Rocket.Chat Community Edition for a conventional Slack-like experience, or Zulip if you value exceptionally organized discussions.

And there's an important distinction here: "the vendor doesn't own our data" isn't the same as "the vendor can never access our data." Self-hosting is the strongest option because you control the server, database, backups, and access credentials.

To truly own your data and ensure complete digital sovereignty, you need an open-source team chat platform that you can self-host on your own hardware or private cloud infrastructure. When you host the server yourself, no third party can claim ownership, monetize, or legally intercept your messages.

The top open-source, self-hosted team chat applications tailored for different team workflows include:

• Mattermost : The closest open-source alternative to Slack in terms of user interface and experience. It is heavily optimized for engineering, development, and high-security DevOps teams. It features robust markdown support, file sharing, incoming/outgoing webhooks, and enterprise-grade compliance. You can deploy the free starter edition via Docker on your own server with no artificial user limits . Learn more on the Mattermost Self-Hosted Overview.
• Rocket.Chat : A feature-rich, highly customizable communication platform that excels in digital sovereignty and omnichannel integrations. It functions as a powerful replacement for Slack or Microsoft Teams, allowing you to manage internal team channels alongside live customer chat. It supports extensive customization and can be fully air-gapped or self-managed. Check out the deployment details via the Rocket.Chat Documentation.
• Element (Matrix Protocol) : Best if your priority is a decentralized, end-to-end encrypted architecture. Rather than a single monolithic server, Matrix operates like an open network (similar to email) where your data lives on your chosen homeserver, and Element acts as the polished client interface. It supports secure team messaging, voice/video calls, and cross-organization federation.
• Zulip : Best for asynchronous-first and highly technical teams. Instead of a flat channel stream, Zulip organizes conversations strictly by streams and topics , making it dramatically easier to track, filter, and search through dense technical discussions without losing context. It is completely open-source and free to self-host, with documentation available at Zulip Server Installation.

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

• What is your team size?
• Do you prefer a Slack-like channel layout (Mattermost/Rocket.Chat) or a threaded/topic layout (Zulip)?
• Do you have a preferred method for self-hosting (such as Docker, Kubernetes, or a raw Linux VPS)?

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

1. Zulip — my first choice

Zulip is genuinely open source under the Apache 2.0 license, and its server can be self-hosted.

Its biggest advantage is that it isn't merely "open source" in marketing while putting important functionality behind proprietary components. Zulip's software is particularly well suited to organizations that want to keep their communications on infrastructure they control.

The interface is somewhat different from Slack: conversations are organized into streams and topics, which can actually be much better for keeping team discussions organized.

2. Rocket.Chat — best if you want a Slack-like experience

Rocket.Chat is open source and supports self-hosting. More importantly for your specific concern, its terms say that Rocket.Chat does not obtain ownership of Customer Data, and its privacy documentation says that with a self-hosted deployment, Rocket.Chat cannot access your user-generated data.

This would be my choice if your team wants something that feels closer to Slack/Teams.

3. Mattermost — good, but I'd put it third for your requirements

Mattermost has self-hosted/open-source offerings, but its licensing situation is more nuanced than Zulip's. Its documentation describes Team Edition as open source, while its source licensing includes AGPL and commercial licensing arrangements depending on the components.

So if "open source" means genuinely permissive, straightforward licensing, Zulip is easier to recommend.

What I'd choose

For maximum ownership/control: → Zulip, self-hosted

For a familiar Slack-like interface: → Rocket.Chat, self-hosted

The important distinction is that self-hosting matters more than simply choosing an "open-source" brand. If your organization's Zulip/Rocket.Chat server is on infrastructure you control, the vendor isn't sitting in the middle of your message database.

If you tell me your team size and whether you need video calls, mobile apps, SSO, or end-to-end encryption, I can narrow this down to the best 1–2 choices.

To keep 100% ownership of your data, you need an open-source team chat platform that you can self-host on your own infrastructure (such as a private server, local hardware, or an independent cloud Virtual Private Server). Because you control the server, no third party can claim ownership or rights to your messages, files, or metadata.

The top open-source, self-hosted team chat applications include:

• Mattermost (Team Edition) : The closest architectural equivalent to Slack. It is feature-rich, highly focused on engineering and developer workflows, and integrates tightly with tools like GitHub, GitLab, and Jira. When hosted on your own hardware, data sovereignty is entirely yours.
• Rocket.Chat : A powerful, highly customizable enterprise communication platform. It supports channels, direct messaging, video conferencing, and even omnichannel customer service inboxes. It can run completely isolated on a private network or air-gapped infrastructure.
• Element (via Matrix) : Built on the decentralized Matrix open standard. Element offers sleek cross-platform apps with end-to-end encryption enabled by default. Because it operates on a federated model, your data isn't trapped in a single siled database unless you choose a standalone server setup.
• Zulip : Famous for its unique, highly efficient threaded conversation model rather than traditional chaotic chat streams. It is open source (Apache license) and exceptional for technical teams that do a lot of asynchronous remote work.
• Nextcloud Talk : If your team already uses or wants a private file-sync and storage cloud, Nextcloud includes a robust real-time chat, audio, and video calling app built directly into its open-source ecosystem.

If you want to narrow this down, let me know:

• How large is your team?
• Do you have server hardware or a preferred cloud host ready to deploy to?
• Do you prefer a Slack-like channel layout or a threaded conversation style?