The company says customers' credit card and payment information was not stolen. But names, phone numbers, email addresses, billing and shipping addresses, the last four digits from credit cards and more may have been accessed in the attack.
Zappos CEO Tony Hsieh alerted employees and customers of the cyber-attack in an email on Sunday that is posted on the company's website.
Zappos is contacting customers by email and urging them to change their passwords.
Zappos said the hacker gained access to its internal network and systems through one of the company's servers in Kentucky. Zappos is based in Las Vegas. It is owned by Seattle-based Amazon.com Inc.