The Washington, D.C-based company said it was working with law enforcement officials to investigate the attack and was contacting customers in nearly all of the countries where it operates.
A banner on its website Friday evening read: "important notice for customers. If you haven't already updated your LivingSocial password, please update it now."
In an email to customers, company CEO Tim O'Shaughnessy requested that users reset their passwords, and he reminded them to disregard any emails claiming to be from LivingSocial that seek personal or account information.
"The security of your information is our priority," he wrote. "We always strive to ensure the security of our customer information, and we are redoubling efforts to prevent any issues in the future."
The company did not immediately explain how the hacking occurred, except to describe it as a cyberattack that "resulted in unauthorized access to some customer data from our servers."
It's the latest bad news for LivingSocial, which offers deals on everything from restaurants to spa sessions. Last November, the company announced it was cutting 400 jobs worldwide, or about 9 percent of its workforce, as the deals marked continued to face challenges.
In recent years, online deals have gone from fad to a much-copied business model that's easy to set up but difficult to sustain. LivingSocial is one of the largest of the online deals companies.
MORE HBJ HEADLINES
Our new comment system is not supported in IE 7. Please upgrade your browser here.