Citigroup said computer hackers breached the bank's network and accessed the data of about 200,000 bank card holders in North America.
The company said the names of customers, account numbers and contact information, including email addresses, were viewed in the breach, which the Financial Times said was discovered by the bank in early May.
However, Citi said other information such as birth dates, social security numbers, card expiration dates and card security codes (CVV) were not compromised.
Citigroup joins a growing list of companies that have suffered cyber attacks.
Data storage firm EMC Ltd this week offered to replace millions of electronic keys after hackers used data from its RSA security division to break into the network of arms supplier and information technology provider Lockheed Martin.
Sony has reported several attacks, including one in which hackers accessed the personal information on 77 million PlayStation Network and Qriocity accounts.
Google last week revealed a major attack on its Gmail accounts targeting, among others, senior U.S. government officials that it said appeared to originate in China.