Assassin’s Creed is the first title to be made available for pre-purchase, and will be able to be downloaded from April 9th.
Tom Clancy titles, the IL-2 Sturmovik series, Far Cry and games from the Heroes of Might & Magic franchise will also be made available in the next few weeks.
"Ubisoft will be delivering the largest library of titles available from any single publisher or developer to Steam," said Jason Holtman, director of business development at Valve.
"The incredible selection of high quality games from Ubisoft expands the range of genres offered to the millions of PC gamers logging on to Steam every day."
Major publishers already signed up to Steam include THQ, Activision, Take-Two, Capcom, Sega, Codemasters and Eidos.