Worldwide, Microsoft has sold more than 19 million units, including the U.S. numbers.
The U.S. milestone, according to Microsoft, is significant because it's an indication of Xbox 360's future market performance. "History has shown us that the first company to reach 10 million in console sales wins the generation battle," Don Mattrick, senior VP of the interactive entertainment business in the entertainment and devices division at Microsoft, said in a statement.
The Xbox got a significant market boost following the April 29 release of "Grand Theft Auto IV," which had the hottest debut in the history of video games. Gamer maker Take-Two Interactive Software sold 6 million units worldwide in the first week. The company makes versions for the Xbox and Sony's PlayStation 3.
GTA IV also helped boost the subscriber base to Microsoft's Xbox Live online gaming service to more than 12 million this month, the company said. The service's membership has doubled over the last year.
Microsoft said on its Gamescoreblog last week that retailers reported that more than 60% of all GTA games sold in the first week were for the software maker's Xbox 360 video game console. This would lead one to believe that the remaining 40% were for Sony's PlayStation, but the consumer electronics company has been mum on GTA's impact.
Microsoft, however, said its own data shows that Xbox 360 sales at retailers went up 54% week over week as a result of GTA's success the first week. In addition, about 40% of the people buying new Xboxes also bought a copy of GTA IV.