Twitch, which is owned by Amazon.com, has reset users' passwords and stream keys and disconnected accounts from Twitter and YouTube. Users will need to set up a new password the next time they log in, it said.
In a brief blog post, Twitch didn't say how many accounts were affected, nor did it say exactly what data was accessed, referring only to "user account information." A spokesman for the service declined to comment further.
Twitch is recommending to users that they change their passwords for any other sites where they use the same or a similar password. "We will communicate directly with affected users with additional details," it said.
Twitch lets users of game consoles like the Xbox One and the PlayStation 4 stream their games online so others can watch. The company said earlier this year it had 100 million monthly viewers.
It was bought by Amazon last year for almost $1 billion.
Zach Miners covers social networking, search and general technology news for IDG News Service. Follow Zach on Twitter at @zachminers. Zach's e-mail address is zach_miners@idg.com