Gears Of War 3 has sold over three million copies worldwide in its first week on sale, and oushed the franchise lifetime sales above the one billion dollars mark.
The title is the first 360 exclusive to do so.
"Gears of War was the first killer app for Xbox 360, and the series has continued to contribute directly to the overall success of the platform ever since," said Phil Spencer, Microsoft corporate VP.
"Gears of War 3 kicked off the biggest Christmas period in Xbox history, setting the standard for the kind of amazing, action-packed experiences gamers should expect on Xbox 360 this year."
The game, developed by Epic Games, was the year's biggest launch so far, scoring the title of seventh biggest Xbox 360 game launch of all time. Around 22,000 retailers globally held special midnight events to support the game.
The game has also scored well with critics, earning a Metacritic score of 91.