'I can't believe I won the Girls' Junior championship,' said Park, 14, who played in her second U.S. Girls' Junior. 'It's a dream come true.'
Park, who was born in Korea but moved to the United States when she was 13, became the second youngest winner of the tournament. Only Aree Wongluekiet, who was 13 when she won in 1999, was younger.
Park went 1-up at the par-4 third after Tangtiphaiboontana two-putted from 20 feet. At the sixth, Tangtiphaiboontana again struggled with the putter and conceded the hole to Park.
Tangtiphaiboontana faltered with the putter at the ninth where she failed to convert from seven feet. Park moved to 3-up around the turn and held the advantage as Tangtiphaiboontana missed another putt at the 10th that would have given her the hole.
'When I missed on 10, I was like, 'Whatever, I still have eight holes left,' said Tangtiphaiboontana.
The 17-year-old Tangtiphaiboontana had another chance to cut the deficit at the par-5 11th but she left her 10-foot putt short. At the 12th she had another birdie opportunity but her putt went right.
'I figured I'd come out and play my game,' said Tangtiphaiboontana, who made the cut for the first time in three appearances at the U.S. Girl's Junior. 'If it's not my day, it's not my day.'
Park moved 4-up on Tangtiphaiboontana with an eight-foot putt at the 13th. Park secured the championship with a putt of equal length at the 15th to become the first medalist to win the title since Beth Bauer in 1997.
Both Park and Tangtiphaiboontana earned entries into this year's U.S. Women's Amateur, which will take place at The Sleep Hollow Country Club in New York from August 12-17. Park has committed to play but Tangtiphaiboontana will most likely opt to skip the event for a vacation.
Last November, Tangtiphaiboontana lost both her parents in a car accident. Tangtiphaiboontana, who was in the automobile at the time of the accident, was accompanied by her brother this week.