From their blog:
WhatsApp has always prioritized making your data and communication as secure as possible. And today, we’re proud to announce that we’ve completed a technological development that makes WhatsApp a leader in protecting your private communication: full end-to-end encryption. From now on when you and your contacts use the latest version of the app, every call you make, and every message, photo, video, file, and voice message you send, is end-to-end encrypted by default, including group chats.
Read more here.