WhatsApp’s newest version (ver. 2.19.20) will now let you restrict access to the app using either Face ID or Touch ID—depending on what kind of iPhone you have. This feature can be activated from its Privacy menu within the Account Settings.
But there are a few things you need to know. It can’t be set on a per-chat basis, though. Also, if you have message previews turned on, you’ll still be able to see and reply to messages without having to unlock the app. You can also answer calls outside of the app. But if you want to use it, you’re going to need to unlock it with either feature. There isn’t any word yet if or when Android’s getting an equivalent feature.