The 21-year-old Larson became the first California native to win a Nationwide race at the 2-mile oval, but only after surviving a three-man derby in the final laps.
Harvick finished just 0.342 seconds behind after barely missing on repeated attempts to slip underneath Larson for the lead.
One of NASCAR's most promising young drivers, Larson is from Elk Grove, Calif., near Sacramento.
Larson had finished second in five previous Nationwide races, but never won. Busch held him off at Bristol last week.
Joey Logano was fourth, and Elliott Sadler finished fifth.