Hackers have broken into Citibank's computers, stealing names, account numbers and contact information of customers, the bank said in a statement.
Citibank discovered a data breach in early May, according to the Financial Times, but did not disclose the details about the hack.
In a statement to the AP, Citigroup says it has taken measures to prevent these types of attacks and that it is informing affected customers about the incident. The hack affected about 1% of the company's 21 million customers.
The Citibank hack follows several other high-profile attacks in recent months. Most infamously, a series of hacking attacks on Sony took down the company's PlayStation Network for about a month, and hundreds of thousands of user profiles containing personal information were exposed.