The Facebook-owned app now outperforms the social network's own Messenger mobile app, which has 800 million monthly users.
The company said 42 billion messages and 250 million videos were sent over WhatsApp daily.
But one analyst said WhatsApp still trailed behind local competition in some key markets.
"There are big markets where WhatsApp isn't the dominant player," said Jack Kent, mobile analyst at IHS.
"WeChat in China has more than 500 million users, while Line is popular in Japan, and Kakao Talk is big in South Korea. But WhatsApp is certainly the most internationally successful.
"Part of that is down to its pure focus on communication, providing low cost chat that is very reliable.
"Other apps have focused on monetisation, games and stickers but WhatsApp's appeal is that it is light on monetisation and it has now dropped its small annual fee completely." Read more on bbc.