The MLS champions made quick work of the Costa Ricans with three goals in the first half hour.
Los Angeles opened the scoring in just the sixth minute. Jack McBean slipped his marker and put a downward header onto Rafa Garcia's corner kick for the opener.
Chandler Hoffman doubled up in the 19th. Michael Stephens played a one-time cross from a McBean pass, and Hoffman was in place to redirect the ball in off the post.
Laurent Courtois added the third and final goal in the 30th minute, the veteran picking up a loose ball 25 yards out and slamming it home.
The win moves LA to 3-0-0 and ensures the Galaxy passage to the knockout round. They will still have to travel to El Salvador on Oct. 24 to play Isidro Metapan in the final group stage game.