People are unable to send or receive chats or even load up conversations, according to users.
And there doesn't seem to be any easy way of fixing the issue, which is affecting many of its users and is likely a problem with its servers.
Problems with the app surged over the last hour, according to the website Down Detector.
Those problems were particularly focused in western Europe, the East Coast of the US and South America, according to the same website. But that may simply be a result of timezones, and the problems could be happening elsewhere.
Users are reporting problems with sending and receiving messages, logging in or loading the app at all.
The chat app breaks less than some of its competitors. But it has run into problems in the past – including a major outage that hit it on New Year's Eve in 2015.
Unlike other popular apps, WhatsApp doesn't maintain a status page to alert users to when it is broken. It has a Twitter account intended for doing so but hasn't used it since 2014, despite having more than two million follows.