The bank said it recently discovered during routine monitoring that account information for about 1 percent of customers was viewed.
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 hackers were able to gain access to Citi's Account Online service to view customer names, their account numbers and contact information including email addresses.
They weren't able to gain access to social security numbers, birth dates, card expiry dates or card security codes.
The Citigroup online security breach comes on the heels of the high profile hacker attacks against multiple Sony Corp. sites. An attack against Sony's PlayStation Network site affected more than 100 million online accounts and forced the site to temporarily shutdown.
Sean Kevelighan, a spokesman for Citi's North America Consumer Banking division, said in an emailed statement that Citi is contacting customers who were affected and is putting in procedures to prevent the security breach happening again.
"For the security of these customers, we are not disclosing further details," he said.
The hacker attack was first reported by the Financial Times.