Johnson joins DC United from Seattle
The 29-year-old Johnson has led the Sounders in scoring the past two seasons and ranks second in club history with 23 regular-season goals.
The Sounders received allocation money as part of the trade, which was announced on Tuesday.
Johnson ranks second in Sounders' history with eight game-winning goals and is the club's all-time leader with three goals in the Major League Soccer Cup Playoffs. He was an All-Star selection in 2012 and named the MLS Comeback Player of the Year.
Johnson also made 17 appearances for the U.S. Men's National Team in 2013.