Choi's second-place finish was nonetheless the most accomplished runner-up finish ever by an amateur in this championship.
Park, who had led after 36 holes in 2016, started the final round two strokes behind Feng, the leader after each of the first three rounds.
Park's play was a one-way express lane to victory on Sunday, where she paused only to give a wave and a bow toward Trump on her way from the 18th to the scoring tent. Park had an 11-under par total of 277, two better than Choi.
"Well, it would be nice if I could get the money, but I think my primary goal was to come here and compete", Choi said.
Park was in contention at last year's Open, but she finished with a pair of 74s in the final two rounds. Third-round leader Patrick Rodgers shot 70 to finish second.
Yang, a top-four finisher in the last three Women's Opens, rolled in a 20-foot left-to-right birdie curler at the 17th hole, while No. 2-ranked amateur Hye Jin sank a long birdie putt at the par-three 16th hole.
The 17-year-old Choi bogeyed the first but fought back with gains at the eighth, 11th and 16th. That's Cristie Kerr, who was five shots off the lead. "Coming into this week I had no expectation at all". "So fans gave me that nickname previous year that I just focus on attack and aggressiveness". "If Plan A doesn't work I'll go to Plan B", Feng said of her final round.
"I just have to stay focused and play my game".
Overnight leader Feng Shanshan slipped back into a tie for fifth after a 75.
Park, a rookie on the LPGA Tour, entered the weekend at just one under after shooting 73 and 70 on Thursday and Friday.
Feng shot a 1-under 71 to reach 9-under 207 at the president's Trump National Golf Club.
That put the Olympic bronze medal victor two clear of Korean trio Hye Jin Choi (69), Jeongeun Lee (69) and Amy Yang (71). "I am not really focusing on the prize money for now". "An amateur player is co-leading for the first time in many decades - very exciting!"
Yang, who had three birdies on the back nine for a 33, and Choi, an amateur teenager, both finished with a 70.
South Korean women's golf domination presented a new face on Sunday in U.S. Women's Open victor Park Sung-hyun, whose long-hitting and ultra-aggressive style set her apart from many compatriots. She added four birdies and two more bogeys in her final 15.
Bad weather then forced a lengthy delay, with some players still to finish their opening round when darkness forced an end to proceedings shortly after 8.30pm local time.
The par meant that Feng needed an eagle on 18 to tie.
A birdie on the par-five first proved to be her last of the day, but none of those with holes remaining look likely to catch the 27-year-old.