The 31-year-old has penned a two-and-a-half-year deal in the United States capital as he leaves the Premier League after 280 appearances in which he scored 86 goals.
He arrived in England 10 years ago, joining Aston Villa from Belgian side Genk before going on to score 42 times in 89 Premier League appearances for the Midlands club.
His prolific spell there earned him a big-money move to Liverpool, but he could only manage nine league goals for the Reds before making a switch to Palace in 2016.
Although he scored 15 top-flight goals in his first campaign at Selhurst Park, the Belgium international never quite recaptured his Aston Villa form and has now made the move to MLS.
D.C. United sits bottom of the Eastern Conference, with former Manchester United striker Rooney having taken over in July for the sacked Hernan Losada.
Rooney is glad to have brought a player of Benteke's quality in. "Christian is a top player who has played at the highest level for a long time," he said.
"His experience and ability to score goals and help the team will be invaluable. It's exciting for the team and myself to get him in and playing. He will make a huge difference."