Net Marketing, Omiai's parent company, said its servers were accessed without authorisation in April. The data included identity cards, driving licences, insurance cards and passports, but no credit card details were leaked.
This is not the first time dating site user data has been leaked. Ashleymadison.com, an extramarital affairs website, was hacked in 2015, leaking the data of 37m users.
As of April, Omiai had 6.8 million registered users, according to a monthly report by the Net Marketing company. The app makes most of its money by charging men for its services, which start at $37 a month, while offering them free to women.
NET Marketing is listed on the Tokyo Stock Exchange and currently has a market capitalization of approximately US $70 million.
Japan's online matchmaking market has more than doubled in size over the past four years and is expected to grow another 70 percent to 106 billion yen by 2025 and 2020, according to a study by Japanese firm CyberAgent.
Omiai has partnered with several local governments in Japan to run matchmaking campaigns and encourage users to move to rural areas after marriage.