Rooney, who is United's all-time top scorer with 253 goals, arrived in the United States on Thursday, and D.C. United announced he has signed a three-and-a-half-year contract.
"It is fantastic to be joining D.C. United at such an exciting time in the club's history with the new stadium opening in just a few weeks," Rooney said, according to the club.
"Moving to America and MLS fulfils another career ambition for me. I have the hunger to be a success here and will give D.C. 100 per cent -- as I have always done for every team I have ever played for.
"When I visited earlier this summer I was really impressed with everyone I met connected with the club, and of course the new Audi Field. Now I can't wait to get on the pitch in a United shirt and join my new teammates to bring success to this club."
The club is in the last place in the Eastern Conference and no team has fewer points in the league. United has won just two of its first 12 games, scoring 19 goals in the process.
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