The 23-year-old carded a final-round five-under-par 67 to finish on 11 under at the Trump National in New Jersey.
Compatriot Choi Hye-Jin was two strokes behind in second, while China's Shanshan Feng, who held a one-shot lead overnight, went round in 75 to finish tied for fourth, five strokes adrift.
A South Korean has won the US Open seven times in the past 10 years.
Park told Fox Sports: "I did not have the best first and second rounds so I wanted to believe in myself in the remaining rounds. Trusting myself definitely helped."
England's Charley Hull finished 11 strokes behind Park on level par, with Scotland's Catriona Matthew a further six shots back.
South Korea's Park Sung-hyun won the US Women's Open by two shots to claim her first LPGA title in her debut season.