Boswell, 34, was in the middle of the fourth season of his second stint with DCU, after beginning his career with the club in 2005.
Overall, Boswell made 198 league appearances (regular season and playoffs) for DCU, scoring 10 goals and recording eight assists.
“Bobby has played an integral role in the history of this club,” United general manager Dave Kasper said in a club release.
“He’s one of the fiercest competitors this league has ever seen and his dedication for winning has had a profound and consistent effect on our locker room for the seven seasons he has played for D.C. United. He’s a true professional and we wish him luck in his next chapter in Atlanta.”
Boswell, a two-time MLS Best XI selection (2006 and 2014), will help bolster Atlanta's depth at center back down the stretch, and will slot in behind first-choice paring Leandro Gonzalez Pirez and Michael Parkhurst.