Washington signed Harris on Wednesday after Josh Howard was lost for the season when he tore the anterior cruciate ligament in his left knee in Monday's game.
Harris was averaging 26.3 points and 9.8 rebounds in 28 games for Rio Grande Valley in the NBA D-League.
The 6-foot-6 Harris played in two games earlier this season with the Houston Rockets, averaging 2.5 points and 1.5 rebounds.
In 2007-08, he played 17 games with the Rockets, averaging 3.7 points and 3.2 rebounds.
Harris will be in uniform Wednesday for Washington's game against Memphis.