Threema Alternatives for Linux

Telegram is a free cloud-based instant messaging platform.

Slack is a cloud-based team collaboration tool.

Tox is a peer-to-peer instant-messaging and video-calling protocol that offers end-to-end encryption.

Signal is the most scalable encryption tool.

Wire is a cross-platform, encrypted instant messaging client created by Wire Swiss. It is available for iOS, Android, Linux, Windows, macOS and Web browser clients.

Bitmessage is a decentralized, encrypted, peer-to-peer, trustless communications protocol that can be used by one person to send encrypted messages to another person, or to multiple subscribers.

Bit Chat is a secure, peer-to-peer, open source instant messenger with end-to-end encryption.

Zulip is a chat and collaborative software.

Wickr is an instant messenger application.