Ahead by four strokes at the start of the round, Park birdied the ninth and 10th holes to extend her lead.
Inbee Park has won the U.S. Women's Open to make history with titles in the year's first three majors.
The world's top-ranked player finished at 8 under to win by four strokes. Her 2-over 74 in the final round Sunday was more than enough, with Sebonack's trying conditions keeping any rivals from making a run. Only three players were under par for the tournament.
Fellow South Korean I.K. Kim also shot 74 for her second runner-up finish at a major.
No player had ever won the first three majors in a year with at least four.
Ahead by four strokes at the start of the round, Park birdied the ninth and 10th holes to extend her lead. She has won six times already this year, including three straight tournaments. Park added a second U.S. Women's Open title to another historic victory in 2008, when she became the event's youngest champion at age 19.