DeRosario, who had entered the game as a substitute in the 85th minute, raced down the line past defender Adam Frye and steered the ball into the far left of the net with a 30-yard blast.
The goal kept the Galaxy winless in four MLS Cup finals appearances.
Although it was a well-played match, the tone was muted by the huge numbers of empty seats--a direct result of having two West Coast teams playing at an eastern site and having neither Chicago nor Columbus in the finals.
Those that showed up were treated to an elegant passing game and 90 solid minutes of attacking soccer.
Los Angeles struck first in the 21st minute with a deadly volley by Luis Hernandez against the run of play. Hernandez collected a long pass from Greg Vanney at the top of the box and one-timed it past Joe Cannon.
San Jose defender Jeff Agoos had badly misplayed the pass, allowing Hernandez to streak around him and put the Galaxy up, 1-0.
Until that point, the Quakes had been fairly dominant, steadily challenging goalkeeper Kevin Hartman, though not really getting much out of the right flank, where the absence of Manny Lagos (hamstring) was sorely felt. His replacement, Ramiro Corrales, looked unsure of where he was supposed to be much of the match.
But the Quakes proved resilient and kept the pressure on. That effort finally was rewarded when Landon Donovan cracked what had been a solid back line in the 43rd minute off a solid feed from Ian Russell that split two defenders.
Catching the ball inside the area, Donovan half-volleyed a 16-yard strike past a helpless Hartman to draw the teams level.
San Jose came out of the blocks quickly in the second half, with Ronnie Ekelund taking only two minutes to force Hartman to stop a scorching shot.
Agoos nearly scored a minute later when his free-kick, following Paul Caligiuri's tackle of Russell, came off the goalpost. Hartman was forced to slap away the rebound and watched Ronald Cerritos sky the ball over the net.
Cerritos uncorked another brilliant shot in the 57th minute that just narrowly missed left.
The Galaxy had their chances, but they were just fewer in number. Cobi Jones looked dangerous on more than one occasion as he showed he still had the speed to bedevil defenders. But a lack of support from a strangely quiet Mauricio Cienfuegos hurt.
There was a surreal tone to the match. The most energetic cheering section was for a team that was not even there. The Chicago Fire's BarnBurners came out in force to cheer for the Fire, and against the Galaxy, who had knocked Chicago out of the Cup. In fact, during the rare moments the BarnBurners were silent, so was the stadium.