"WhatsApp now has 900 million monthly active users," WhatsApp co-founder Jan Koum said in a Facebook post.
In February last year, WhatsApp was acquired by social networking site Facebook, its biggest buyout till date, for a whopping USD 19 billion.
In his post, Koum also tagged Facebook CEO Mark Zuckerberg, who promptly put up a post congratulating him on the milestone.
Facebook COO Sheryl Sandberg also congratulated Koum in a post saying "900 million people now use WhatsApp every month to stay in touch with family and friends around the world.
Congratulations to Jan Koum and everyone at WhatsApp on reaching this incredible milestone ? and for still finding time to make us all laugh."
Previously, WhatsApp has said its service has seen strong uptake, specially across developing nations like Brazil, India and Russia.
In November, WhatsApp had said India accounted for 70 million of its active users, which was over a 10th of its global users back then.
WhatsApp competes with messaging apps like LINE, Viber and Hike in India.