More than 92,000 accounts held by Citigroup's Citi Cards Japan have been stolen and sold illegally to a third party.
Lost customer details include account numbers, names, addresses, phone numbers, date of birth, gender and account opening dates.
Citi Cards Japan (CCJ) said PIN numbers and card security codes (CVVs) were not compromised.
"CCJ immediately reported the inappropriate sale to the relevant authorities and the police and has been cooperating fully with the investigation," it said in a statement.
"While the risk of fraud is minimal due to the absence of security information, CCJ has placed internal fraud alerts and enhanced monitoring on all accounts identified and no unusual or suspicious credit cards transactions relating to these customers have been detected at this point.
The company had informed customers by mail and ha posted a notice on its web site.
It said it would re-issue cards on an as-needed basis.
The breach is the second in almost as many months for Citigroup, which lost some 200,000 records of North American customers after it was hacked in June.
That attack was less severe because birth dates, social security numbers, card expiration dates and CVVs were not compromised.