The Timbers started the game strongly and could have taken the lead through Diego Valeri as he lined up a free kick from 25 yards which was deflected and went inches past the post.
However, it was Montreal who took the lead when Justin Mapp raided down the right and squared the ball to Andres Romero. The midfielder took a neat touch past a defender and buried his effort past Donovan Ricketts.
The visitors were handed the chance to take the lead when Hassoun Camara brought down Valeri in the box and a penalty was awarded. Will Johnson took the spot kick and fired the ball past Bush.
Montreal netted an equaliser just before the break when Maxim Tissot's effort from inside the area managed to beat the outstretched hand of Ricketts to nestle in the bottom corner.
In the second half, both sides had half chances to take the lead as Romero scuffed his shot wide from inside the box, while at the other end Urruti blasted his volley over the bar.
Portland won the game in the 82nd minute when Valeri's curling shot from outside the box crept under Bush and into the net.