Mobile messenger app Line had drawn over 400 million subscribers as of Wednesday. It was developed by NHN, which runs Korea's biggest portal site Naver.
Line has achieved the milestone in just 33 months of its launch in June 2011. It hit the 300-million mark last November.
Rivals include American-made WhatsApp and WeChat, run by Tencent, one of China's biggest portals. Facebook recently acquired WhatsApp and access to its 450 million subscribers.
WeChat is quickly growing in strength in the U.S. and Europe, but is absolutely dominant in China. In contrast, Line is popular in Japan and Southeast Asia.
Line has over 10 million users in each of the following countries: Japan (50 million), Thailand (24 million), Indonesia (20 million), India (18 million), Taiwan (17 million), Spain (16 million), Korea, Malaysia, the U.S. and Mexico (10 million each).