According to his post, he has decided to "start a new chapter" in life: "a non-profit focused at the intersection of nonprofit, technology and communications."
The Stanford alumnus founded WhatsApp with Ukrainian Jan Koum in 2009. In 2014, it was acquired by Facebook in a $19bn deal.
WhatsApp co-founder Brian Acton leaving company
What was Acton doing at WhatsApp?
Acton, currently worth $6.5bn according to Forbes, was responsible for leading the engineering division at WhatsApp. According to sources, he was behind the platform's push into encryption.
Both Acton and Koum are former Yahoo employees. Ironically, Acton was rejected by Facebook before he started WhatsApp.
What's next for Acton?
Acton called himself fortunate "to have the flexibility to take new risks". He will now "focus and execute" (on) his NGO, about which not much is known yet. "It's something I've thought about for a while...I'll have more to share in the coming months."