Despite its enormous user base, WhatsApp can often be slow to introduce the most hotly-anticipated new features. A Dark mode is still in the works, but fingerprint unlocking already came to iOS a few months ago. It's now finally available in the latest beta channel update to the Android app.
When you've installed version 2.19.221, you should see the Fingerprint lock option in Settings > Privacy, but it may also be linked to a server-side switch, so don't panic if you don't get it straight away. You might be able to force it by deleting and reinstalling the app, but make sure you back up your chats if you do!
Once you have the option, it's fairly straightforward. WhatsApp will talk to your phone and allow your saved biometric credentials to work with the app, then you'll be asked whether you want it to lock as soon as you close it or wait for 30 seconds/1 minute. There's also an option that allows you to hide notification content when locked.
To update WhatsApp beta to the latest version, head to the Play Store or download it from APK Mirror. The feature will make its way to the stable version in due course.
Available to more with new update
As many are pointing out in the comments, another new update has landed that brings this feature to even more users. With beta version 2.19.222, most people if not everyone should have fingerprint unlocking now.
In a less welcome move, you'll also see 'WhatsApp from Facebook' at the bottom of the settings change. Don't remind us!
You can download v2.19.222 from APK Mirror if it's not yet available to you on the Play Store. But beware, it won't work unless you were already part of the beta.