Giacomo, a 50-1 choice, broke away from a pack down the stretch and beat Closing Argument by one-half length. Afleet Alex was another half length back in third.
The time for the 1 1/4 mile race was 2:02 3/5.
It was Giacomo's second win in eight starts. It was his first victory since October of 2004.
The son of beaten 1994 Kentucky Derby favorite Holy Bull was fourth in his last previous race, the Santa Anita Derby.
Giacomo paid $102.60, $45.80 and $19.80; Closing Argument $70.00 and $24.80; and Afleet Alex $4.60 to show.
Nick Zito had five starters in the 20-horse field, including the favorite in Bellamy Road, who finished seventh -- 6 3/4 lengths behind the winner.