The Seattle Sounders were sent packing as Jason Kreis's side showed its quality to claim first place in the Western Conference.
RSL took the lead in the 41st minute, when Javier Morales' free kick was deflected off the wall and fell to Kyle Beckerman, who did brilliantly to side-volley a high shot into the ceiling of the net.
Having scored just before halftime, RSL doubled its lead just after through Robbie Findley. The World Cup veteran had his initial effort saved by Michael Gspurning, but Alvaro Saborio was trailing, and kicked the ball back at Findley, who finished with his second effort.
Seattle's best chance came in the 82nd minute, when Obafemi Martins rounded Nick Rimando but fired his shot into the side netting.
Real Salt Lake moves into first in the West, with 30 points from 17 games. Seattle, with 21 from 14, is back in sixth, out of the last playoff spot on goal difference.