Citigroup Inc said computer hackers breached the bank's network and accessed the data of about 200,000 bank card holders in North America.
Citi 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.
"We are contacting customers whose information was impacted. Citi has implemented enhanced procedures to prevent a recurrence of this type of event," Sean Kevelighan, a U.S.-based spokesman, said by email.
"For the security of these customers, we are not disclosing further details."
In the brief email statement, Citi did not say how the breach had occurred.