Yahoo Japan fears that up to 22 million user IDs may have been unlawfully siezed, in an attempted hack last week.
At around 9.00pm local time on Thursday, Yahoo Japan's administrative system was attacked, with the Internet giant unable to identify how exactly the system had been penetrated.
Access to the service was reportedly cut, once the attack was discovered.
According to the firm, the suspected stolen information did not contain passwords or data that could be used to hack accounts but, as a precaution, Yahoo Japan has now recommended its 200 million users change their login details.
"We don't know if the file was leaked or not, but we can't deny the possibility, given the volume of traffic between our server and external terminals," Yahoo Japan told the AFP.
Yahoo Japan is the top search engine in Japan, holding over 50 per cent of the market share, compared to Google's 40 per cent cut.
This attack is the latest in a long line of recent hacks, closely following the Syrian Electronic Army's breach of the Financial Times' website and Twitter feeds.
Yahoo, meanwhile, looks set to acquire micro-blogging service Tumblr for £725 million, in Marissa Mayer's biggest piece of business to date.