Seeking its third consecutive Major League Soccer championship, the Galaxy will open the two-leg Western Conference semifinals Sunday at StubHub Center.
The second semifinal game would be the following weekend in Utah. The Galaxy was 2-1-0 against Real Salt Lake during the regular season.
The Galaxy overall finished the regular season in third place in the conference with a record of 15-11-8 and 53 points, four points behind the first-place Portland Timbers (14-5-15).
Real Salt Lake (16-10-8) finished second in the conference with 56 points and Seattle (15-12-7) — which was winless in its last seven games — finished fourth with 52 points.
Seattle and the fifth-place Colorado Rapids will play a knockout game Wednesday to decide which team plays Portland in the other conference semifinal pairing.
Standing just beyond the 18-yard box, Keane kicked a shot that quickly dived into the turf and then caromed high and past Seattle goalkeeper Michael Gspurning.
The Sounders' goal came in the 30th minute when acclaimed forward Clint Dempsey scored for the first time since joining Seattle in August.
Dempsey initially took a long cross pass from Lamar Neagle that forced Galaxy goalkeeper Jaime Penedo to jump high and out of position.
Then, as Dempsey ran past the goal, he flicked the ball with the right side of his right foot behind Penedo and into the net.
Until Keane scored, it appeared the Galaxy might be haunted by a play in the 42nd minute involving Galaxy defender Omar Gonzalez.
Gonzalez struck a clean header from the six-yard line that appeared to bounce into the goal before midfielder Osvaldo Alonso kicked it back out.
But as Gonzalez raised his hands in jubilation, the officials did not rule it was a score, even though television replays showed the ball appeared to travel at least a foot inside the goal.
The Galaxy's Landon Donovan, meanwhile, will have to wait until next season to set the MLS career regular-season scoring record. The forward remains tied for the record with Jeff Cunningham with 134 goals.
The Sounders' defense kept Donovan relatively quiet. In the 25th minute Donovan took a pass from Keane and made a glancing shot at goal off his left foot from just beyond the six-yard box, but Gspurning easily saved it.
Peltz reported from Los Angeles.