The Marriott said hackers gained "unauthorized access" to the Starwood reservation system since 2014, but the company identified the problem last week.
"The company recently discovered that an unauthorized party copied and encrypted data and took steps to remove it," Marriott said in a statement.
For 327 million people, Marriott says the guest information exhibited includes their names, phone numbers, email addresses, passport numbers, date of birth, and arrival and departure information. For millions of others, their credit card numbers and card expiration dates are potentially endangered.
The Marriott warns it can not confirm whether the hackers have managed to decipher the credit card numbers.
"We miss what our guests deserve and what we expect from ourselves, we do everything we can to support our guests and use the lessons learned to get ahead," executive director Arne Sorenson said in a statement.