The Sounders were the first team to threaten when Leonardo Gonzalez lifted a cross into the box, but Chad Barrett's header went wide of the mark.
Montreal hit back with an opportunity of their own as Dominic Oduro was played though on goal, but Troy Perkins raced off his line to deny the forward with a good block.
The home side went close again as Ignacio Piatti drilled a low effort towards the bottom corner, but Perkins was there to make the save.
Perkins continued to keep the Sounders level just before the break when he made another fine stop to deny a strike from Andres Romero.
In the second half, Lamar Neagle pulled free of his marker in the middle to latch on to Barrett's cross from the right, only to put his header wide of the post.
The match looked to be heading for a goalless draw until Montreal were awarded a free kick and Ciman rose highest in the area to guide Marco Donadel's pinpoint cross past Perkins to secure the win for his side.