Brad Evans and Sammy Ochoa also scored for Seattle, which moved into a second-place tie with idle Real Salt Lake in the Western Conference. The Sounders (12-6-7) have won four of their last five games.
Montero scored his first two goals in the 27th and 34th minutes for his first multi-goal regular-season game since May 9. He completed the hat trick and pushed the Sounders' lead back to two goals. Montero reached 11 goals on the season, the fourth time in four seasons he has reached that milestone.
Juan Agudelo scored a penalty kick as Chivas (7-10-6) went into the intermission down 3-1. Miller Bolanos scored in the 64th minute, but Montero's goal 3 minutes later quieted the comeback. Rosales assisted on all three of Montero's goals.