The Guatemalan, who arrived in MLS this year, scored nine game-winning goals. At 23, he is the youngest player to win the award, which was voted on by coaches, general managers, players and the media.
Ruiz scored more than half of Los Angeles' goals. He edged out finalists Taylor Twellman (New England) and Mark Chung (Colorado).
On Sunday, Ruiz will hope to become the second MVP, after Tony Meola in 2000, to win the league title in the same year.
MLS MOST VALUABLE PLAYER
2002: Carlos Ruiz (Los Angeles)
2001: Alex Pineda Chacon (Miami)
2000: Tony Meola (Kansas City)
1999: Jason Kreis (Dallas)
1998: Marco Etcheverry (D.C. United)
1997: Preki (Kansas City)
1996: Carlos Valderrama (Tampa Bay)