Park, a 14-year-old playing in her first USGA championship, birdied the 18th hole of the 6,391-yard course at Tacoma Country and Golf Club to beat 17-year-old Stephanie Kono 2-up.
The runner-up to the U.S. Women's Open qualifier Courtney Ellenbogen at this year's AJGA Rolex Tournament of Champions will oppose Ayaka Kaneko in the final match. Park, from Buena Park, Calif., is trying to become the fourth-youngest champion in the tournament's history.
Park held a 3-up lead on Kono when she birdied on the 13th hole. But she then let Kono, playing in her sixth U.S. Girls' Junior, back into it by taking a bogey and double-bogey on the next two holes. Park ended the drama by making her birdie on the final hole.
Kono, who had played 43 holes in two matches Thursday to advance to the semifinals, never led against Park.
Kaneko, a 17-year-old who moved with her family from Tokyo to Honolulu in 2002 so she play golf more regularly, never trailed in her match with Michelle Shin before a birdie at No. 18 clinched her 1-up win.