Kim's triumph at Aronimink Golf Course after a scintillating final round 63 put paid to her status as the most prolific winner on the LPGA Tour among active players never to have won a major title.
"I’m so excited. I’m really hiding my tears at the moment. It was amazing that I really won it. So I’m very happy and excited that I got it done," the 27-year-old South Korean said, beaming next to the winner's trophy.
Seven-times major champion Park had beaten Kim for the 2015 title at Westchester Country Club and was only three strokes behind her compatriot going into the final round.
A five-under round might get the job done, she thought.
Park duly delivered with a 65 but it was not enough as overnight leader Kim rose to the challenge with her seven-under-par 63.
"Obviously Sei-young was just much better than anyone else out there today," said Park, who had been chasing a fourth win in the event.
"It was just so hard to believe that she never won a major before because it felt like she won a few," she said of Kim, who has claimed at least one Tour win each season since 2015.
Kim, who was in a tie for second at the 2018 Evian Championship, one of the five women's majors, admitted to struggling with nerves before teeing off on Sunday and kept her eyes averted from the leaderboard during her round.
"I won't lie, I did feel the pressure starting last night," she said. "I actually arrived about 30 minutes later than I normally do at the golf course.
"I did not look at the scoreboard once. I knew In-bee was going to play great. But I had to just focus on playing my own game, one shot at a time."
(Reporting by Ian Ransom in Melbourne; Editing by Peter Rutherford)