Users have flagged the issue on Twitter with the hashtag #WhatsAppDown.
Downdetector.co.uk shows the problem has spread across the globe, with users in the UK and Europe worst off.
Users in other countries including Barbados, India, Panama, South Africa, Spain and Qatar are also unable to access the app on New Year’s Eve.
One wrote: “Messages won’t deliver. Can’t see when people have been online. This is my lifeline for my New Year plans. I’ve had to go old school and text and ring people.”
Another added: “Not working for me in the UK. Refusing to connect with wi fi or 4G.”
The company previously warned the app would stop working on some phones at midnight, but many users are still hours away from 2018.
WhatsApp is used by a staggering one billion people worldwide.
It crashed in November with thousands reporting issues in the UK.
The messaging service also went down in May with no explanation.