In post on Facebook, he said he was “taking some time off to do things I enjoy outside of technology”.
In his statement Mr Koum said: “It’s been almost a decade since Brian [Acton] and I started WhatsApp, and it’s been an amazing journey with some of the best people.
The team is stronger than ever and it’ll continue to do amazing things… And I’ll still be cheering WhatsApp on – just from the outside.”
Stanford alumnus Brian Acton and Ukrainian immigrant Mr Koum co-founded WhatsApp in 2009, before selling it to Facebook in 2014 for $19bn (£13.8bn).
Facebook chief executive Mark Zuckerberg commented on Mr Koum’s post, saying he was grateful for what Koum taught him about encryption “and its ability to take power from centralized systems and put it back in people’s hands.”