Blizzard Entertainment said no financial information appeared to have been breached, but that "unauthorised and illegal access into our internal network" had been detected.
"Some data was illegally accessed, including a list of email addresses for global Battle.net users, outside of China," it wrote in a blog post.
The attack hit the Battle.net service, which gamers use to play Blizzard titles including Diablo III and the massively popular World of Warcraft.
Encrypted passwords, answers to security questions and other information was taken in the raid, the company said.
"For players on North American servers (which generally includes players from North America, Latin America, Australia, New Zealand, and Southeast Asia) the answer to the personal security question, and information relating to Mobile and Dial-In Authenticators were also accessed."
It warned players who use similar passwords for other accounts, including email addresses, to consider changing them to avoid wider security issues.
"We deeply regret the inconvenience to all of you and understand you may have questions. … We take the security of your personal information very seriously, and we are truly sorry that this has happened."
The attack is just the latest in a series of high-profile hacks of popular social networks, websites and gaming services.