Benteke departs Crystal Palace after six years at the club during which he scored 37 goals in 177 appearances.
Benteke still had one year left on his contract with the Eagles but has opted to leave and sign a deal until the summer of 2024 with Wayne Rooney's side.
The 31-year-old scored just four goals in 25 appearances in the top flight for Palace last season but was an instant hit when he first moved to England in the 2012/2013 season with Aston Villa, scoring 19 goals in 34 games that season.
The Belgium international earned a big-money move to Liverpool in the 2015/2016 campaign but left Anfield after just one season.
DC United are currently bottom of the Eastern Conference and will be hoping Benteke's arrival helps them move up the table.
“Christian is a top player who has played at the highest level for a long time," Rooney told DC United's official website.
“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.”
Meanwhile, the team's president of soccer operations, Dave Kasper added: “We’ve been working hard over the last few weeks to bring in talent to bolster our squad.
"Being able to bring in a world class player of Christian’s [Benteke] caliber is a huge testament to what’s going on behind the scenes at the club and our ability to draw top talent globally. We have been extremely active in the transfer window bringing in six players and since Wayne Rooney’s appointment as our Head Coach we believe we have made major steps in putting together a roster that can both make a push to finish this season strong and build a foundation for us moving forward.”
Benteke has 18 goals in 45 caps for Belgium.