Mac users will be able to download Portal, Football Manager 2010 and Torchlight at launch, with more titles added every Wednesday.
"We’re very excited to be bringing Torchlight to the Mac," said Max Schaefer, co-founder of Runic Games, the company behind the title. "Having Steam for the Mac solves so many problems for us as a developer."
Steam, which has around 25 million users, was launched in 2004, allowing PC owners to buy and download games to play on their computer. The library of games soon expanded, and there are now more than 1,000 titles available.
Game studios will be able to use the Steam suite of development tools on the Mac platform to add social-networking and antipiracy features. Steam users who own both a Mac and a PC will be able to download already purchased games to their Apple computer.
Valve has also confirmed that it will make Steam available to Linux users in the coming months.