Seattle (6-4-7, 25 points) moves its unbeaten streak to four games and won its second series of the 2011 season after beating Toronto at home on April 30.
Toronto (2-6-9, 15 points) has now gone seven games without a win.
In the 89th minute, Mauro Rosales was fouled from behind by Ty Harden, setting up a Montero free kick. From 22 yards out, Montero bent his shot around the Toronto defensive wall and into the upper right corner of the goal. It was Montero's third goal of the season.
Sounders FC was forced to play nearly the entire second half a man down after Jhon Kennedy Hurtado received his second yellow card of the match in the 49th minute. The man-down victory was the Sounders' first in the club's history in MLS.
The Reds' closest opportunity came in the 81st minute when Javier Martina had a chance to put one in from inside the right side of the box but his shot ricocheted off the near post and the rebound was defended away for a corner.
Kasey Keller recorded his fifth clean sheet of the season, recording two saves. In the 22nd minute, Maicaon Santos sent a header on goal but Keller easily controlled the ball. A similar event took place in the 75th minute when Nick Soolsma's header found Keller's gloves as well.
The Rave Green started the game fast, as Mike Fucito found an opening off a throw-in on the left side in the second minute but his shot grazed off the far post.
In the 74th minute, Osvaldo Alonso fired a shot from the left side of the box off a quick corner kick but Toronto goalkeeper, Stefan Frei, was able to make the save.
Frei tallied three saves on the night, while Toronto held the advantage in shots over Seattle, 18-10. Seattle held the advantage in shots on goal, 4-2.
In addition to Hurtado's ejection, Toronto's Harden, Danleigh Borman, Mikael Yourassowsky and Oscar Cordon were all issued cautions.