The world number one held steady with a final-round two-over-par 74 to finish at eight-under 280 at Sebonack Golf Club in New York.
She earlier this year won the Kraft Nabisco Championship and the LPGA Championship.
Only Mildred (Babe) Didrikson Zaharias has previously won the first three women's Majors in a season. She did it in 1950, winning the Titleholders Championship, the Women's Western Open and the US Women's Open.
Fellow South Korean I.K. Kim matched Parks 74 and took second at four-under 284 but was never able to really challenge Park.
Ryu So-Yeon was third at one-under 287 with Americans Paula Creamer and Angela Stanford and England's Jodi Ewart-Shadoff tied for fourth at one-over 289.