Login identification and passwords of some Tokyo 2020 Olympic and Paralympic Games ticketholders has reportedly been leaked online.
Japanese news agency Kyodo quoted an unnamed Government official, who said the Tokyo 2020 Organising Committee had launched an investigation into the issue.
The official claimed the leak was "not large".
People who signed up to serve as volunteers for the delayed Olympic and Paralympic Games may also have been impacted, it has been claimed.
The data breach is considered to have been caused by someone accessing a computer or smartphone unauthorised, according to Kyodo News.
Cybersecurity has been a concern prior to the Games.
Six Russian intelligence officers were charged in the United States last year for an alleged global computer hacking operation that included the 2018 Winter Olympic Games in Pyeongchang.
According to US Federal Prosecutors, the Russian officers unleashed a corrupted software system known as "Olympic Destroyer" to disrupt the Opening Ceremony of Pyeongchang 2018.
The British Foreign, Commonwealth and Development Office and National Cyber Security Centre had previously warned the Russian military intelligence service - known as the GRU - had conducted cyber reconnaissance against organisers, logistics services and sponsors with the aim of compromising computer systems and sabotaging the running of Tokyo 2020.
Organisers in Japan claimed there was little disruption to the Games, however, with extensive cybersecurity preparations already in place.
Tokyo 2020 held a programme for 220 "ethical hackers" earlier this year.
The programme was designed to help counter potential attacks.
The breach adds to organisers woes, with spectators having already been banned from attending events in most Games veues.
The state of emergency called by the Japanese Government earlier this month meant all spectators would be banned from all stadiums and venues in Tokyo.
There had been hopes that up to 10,000 domestic spectators would have been able to attend select events at the Games, but the multi-sport event will now largely take place behind closed doors.
Overseas fans had already been barred from the Games.