The Trump Hotel Collection is working with the Secret Service and FBI to investigate a possible payment card breach, its second one in less than a year.
The luxury hotel group is run by Republican presidential candidate Donald Trump and his family.
"Like virtually every other company these days, we are routinely targeted by cyber terrorists whose only focus is to inflict harm on great American businesses," said Eric Trump, one of the candidate's sons, in an email statement. "We are committed to safeguarding all guests' personal information and will continue to do so vigilantly."
News of the breach was first reported by computer security writer Brian Krebs, citing three unnamed sources in the financial sector.
Banks often see signs of possible fraudulent activity and can quickly triangulate to find a so-called common point-of-purchase where all cards were used.
Krebs reported that the cards were used at several Trump Hotel properties over the last two or three months, including Trump International Hotel New York, Trump Hotel Waikiki in Honolulu and the Trump International Hotel and Tower in Toronto.
Last October, the Trump Hotel Collection said malware infected payment card terminals and front desk terminals for about two weeks starting in May 2015.
Merchants have been fighting cybercriminals seeking to steal payment card data from their networks. Although merchants are putting in better defenses, such as payment card readers compatible with the EMV system, upgrades have been slow, particularly for smaller companies.