The 23-year-old Park, runaway favorite to win LPGA Rookie of the Year honors, birdied the par-five 15th to forge ahead and, after co-leader amateur Choi Hye-jin double-bogeyed the 16th, added a six-foot birdie at the 17th for some cushion.
"Well, to be honest with you, I still can not believe that it is actually happening", said Park."It's nearly feel like I'm floating on a cloud in the sky". "It feels unreal to a certain extent".
Park hit a chip from over the green on the par-5 18th to save par and win the $900,000 top prize from the $5 million event.
"It's nearly like I'm floating on a cloud in the sky", Park said. "I mean, I did a good job hanging in right there because my putting was not really that great".
"All I tried to do all four days was actually trying to enjoy the process and then focus on every shot, and give 100 percent on every shot", said Feng, who managed just one birdie each of the final two days.
South Koreans have won six of the past nine US Women's Open crowns and have a shock contender in teen prodigy Choi, trying to become the first amateur to win the US Women's Open in half a century.
In this Associated Press photo, Park Sung-hyun of South Korea celebrates after making par at the 18th hole during the final round of the U.S. Women's Open at Trump National Golf Club in Bedminster, New Jersey, on July 16, 2017. Choi's 8-under 208 is the lowest three-day total of any amateur ever in the field.
Throughout the tournament, the presence of Trump, who arrived on Friday and stayed the rest of the weekend, and the protesters who made their way to the area and even into the premises on Sunday were never much of a distraction for the golfers.
Choi responded with a par on 17 and a birdie on 18 to claim sole possession of second place, thanks to Feng taking a triple bogey on the final hole to fall into a fifth-place tie with Carlota Ciganda and Jeongeun6 Lee.
England's Charley Hull finished 11 strokes behind Park on level par, with Scotland's Catriona Matthew a further six shots back. "But I keep in mind to stick with how I usually play because it was part of my practice and I stuck with it and I did it".
"I was hitting the ball ideal".
Park was in fourth place after three rounds, sitting three strokes behind Shanshan Feng of China.
Feng, who has just one bogey in the tournament, could not buy a birdie in the ideal conditions until the last, failing to convert earlier birdie chances from 10 feet and twice from 12 feet in the closing holes.
Australia's Minjee Lee had a late charge and tied for 11th at the US Women's Open. "If Plan B isn't working I'll go to Plan C, which is just knock them in the hole". "He's a great competitor, and I was kind of shocked he bogeyed 17", said DeChambeau, the 10th first-time victor on the tour this season.
"We have the British Open and the Evian, two majors left". Still a very good experience.
The win punctuated a comeback of sorts for DeChambeau, who capped a string of eight consecutive missed cuts last month in the U.S. Open - where he was 6 over for two rounds.