Information for about 1 percent of customers was viewed.
The hackers were able to see customer names, account numbers and contact information, but they did not gain access to social security numbers, birth dates or card security codes.
Citigroup said it is contacting affected customers.
Citi has more than 21 million credit card customers in North America, according to its 2010 annual report. The New York-based bank didn't say exactly how many accounts were breached.
The Associated Press contributed to this report.