The Ghana-born player, 17, is a reported target for Premiership big guns Manchester United and Chelsea but is not allowed to move to Europe until he turns 18 on June 2 next year and has now been traded.
Adu is expected to be given the chance to occupy his preferred attacking midfield role at the Utah franchise.
DC United president and chief executive officer Kevin Payne told the club's official website: "This was a difficult decision for all of us at DC United.
"Freddy has been a valued member of our team both on and off the field for three years.
"We appreciate that he wanted the opportunity to play a different role on the field than he would have been afforded here and he will apparently get that opportunity in Salt Lake. We wish him nothing but the best."
The trade of Adu, along with goalkeeper Nick Rimando, to Salt Lake City does not mean DC United would miss out entirely if he does make a big-money move in the future.
A statement from the club read: "In return, United receive a major allocation, the rights to goalkeeper Jay Nolly and future considerations, including a share of any future replacement allocation money attributed to Adu, should he be transferred."