Threema is described as 'New mobile messaging app that puts security first. With true end-to-end encryption, you can rest assured that only you and the intended recipient can read your messages' and is a very popular instant messenger in the security & privacy category. There are more than 100 alternatives to Threema for a variety of platforms, including Android, iPhone, Windows, Mac and Linux apps. The best Threema alternative is Signal, which is both free and Open Source. Other great apps like Threema are Telegram, Element, Tox and WhatsApp.
Threema alternatives are mainly Instant Messengers, but if you're looking for
Encrypted Chat Apps or Group Chat Apps you can filter on that. These are just examples - use the filter bar below to find more specific alternatives to Threema.