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.
"I just hope this is not a dream," Park, who earlier this year won the Kraft Nabisco and LPGA championships, told reporters. "I don't want to wake up tomorrow and play the final round again."
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 U.S. 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.
Entering the day with a four-stroke advantage, Park's lead was briefly trimmed to three when Kim birdied the second hole but the runner-up gave a shot right back with a bogey on No. 4.
After bogeying holes No. 6 and No. 7, Park ran off birdies at the ninth and 10th holes to swell her cushion to six shots.
Park has won five times overall this year and three in a row. She took over the top ranking after claiming the Kraft Nabisco Championship in April, then added the LPGA Championship earlier this month.
She will attempt to make it four majors in four tries at the at the Women's British Open in August before competing in September's Evian Masters, which has become the women's fifth major.
"I'm glad that I can give it a try at St. Andrews (for the British Open)," Park said. "That's going to be a great experience. Whether or not I (win it), I'm just a very lucky person."
(Reporting by Jahmal Corner in Los Angeles, Editing by Gene Cherry)