The 23-year-old Park, from Levittown, New York, had an eagle and six birdies on a cloudy day over the Bay Course at Stockton Seaview to complete 54 holes at 16-under 197, a stroke off the tournament record.
Yokomine, the winner of 23 events on the Japan LPGA Tour, flirted with a 59 but parred the par-5 18th for a 61 to tie the course record.
New Jersey native Marina Alex was third at 14 under after a 64. She made a hole-in-one at the par-3 17th. Sei Young Kim, who broke the course record Sunday morning when she finished her second round with back-to-back birdies for a 61, had a 70 to finish fourth at 13 under.
Park earned $262,500, topping her total of $261,096 for her first 49 LPGA Tour events. She won the 2013 NCAA individual title as a freshman at Southern California and helped the Trojans take the team crown.
Brooke Henderson had the best Canadian result, scoring 28th in a 69-round. Anne-Catherine Tanguay finished eight places down, despite a score of 67.