Matrix.org is described as «Matrix defines a set of open APIs for decentralised communication, suitable for securely publishing, persisting and subscribing to data over a global open federation of servers with no single point of control» and is a very popular Group Chat app in the social & communications category. There are more than 100 alternatives to Matrix.org for a variety of platforms, including Windows, Linux, Mac, Android and iPhone. The best alternative is Signal, which is both free and Open Source. Other great apps like Matrix.org are Telegram, Element, Discord and Jitsi Desktop.Matrix.org alternatives are mainly Group Chat Apps but may also be Encrypted Chat Apps or Video Calling Apps. Filter by these if you want a narrower list of alternatives or looking for a specific functionality of Matrix.org.
