Threema

https://threema.ch/en

From their about page:

Threema is the world’s best-selling secure messenger. It sets itself apart from others in the following respects:

• Privacy as a guiding principle: Threema is designed to generate as little data on servers as technically possible.
• Full anonymity: You are not forced to provide any personal information (such as your phone number or email address) in order to use Threema.
• Open source: To provide full transparency, the Threema apps are open source. Anyone knowledgeable enough can confirm Threema’s security on their own.
• Optional contact synchronization: It’s not necessary to grant access to the address book in order to use Threema.
• Comprehensive encryption: In Threema, all your communication is end-to-end encrypted – not just text messages and voice and video calls but also group chats, media files, and even status messages.
• Rich functionality: Threema is versatile and feature-rich.

Find out how Threema stacks up against other messengers like WhatsApp, Signal, or Telegram in the comprehensive messenger comparison and the in-depth comparison of Threema and WhatsApp.

With Threema Work and Threema Gateway, Threema also provides solutions for secure communication in business contexts.