The 32-year-old forward signed a three-and-a-half-year contract to bring to an end a disappointing second spell at his boyhood club after just one season.
“Moving to America and MLS fulfils another career ambition for me,” he said after signing.
“I’m not here to see out the last few years of my career, I’m here to compete, I’m here to win and that’s the way I’ve always played and will always play.”
Rooney made his debut on July 14 against Vancouver Whitecaps on the day DC United opened their new stadium, Audi Field, and was named captain after just three matches.
He was named MLS player of the month for October after helping his side to the play-offs and ended the season with 11 goals in 29 matches.
Towards the end of his second season, with two years still remaining on his contract, he agreed a deal to become a player-coach at Derby.
He signed off his final campaign at DC United with another play-off appearance, finishing with 12 goals in 21 matches.