The world's top-ranked player finished at 8 under to win by four strokes. Her 2-over 74 in the final round 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.
"I'm just very glad to put my name in history," Park said.
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.
So Yeon Ryu, the 2011 U.S. Women's Open champion, shot 72 to finish third at 1 under. South Korean players took the top three spots and have won the last five majors.
With lashing wind and devilish greens, Sebonack was a classically troublesome U.S. Women's Open course. And once Park built a lead, nobody could mount a charge.
She certainly wasn't going to make enough mistakes to come back to the field. Park predicted Saturday that shooting even par in the final round would be enough, and she sure was right.
All of four players were under par Sunday -- though that was still more than the third round, when only Park accomplished the feat.
Kim birdied No. 2 to pull within three strokes; she couldn't claw closer. And when she bogeyed the fourth hole, the deficit was back to four shots.
Park bogeyed the sixth and seventh, but so did Kim.
The only player to win the year's first three majors was Babe Zaharias in 1950, back when there were only three.
The women now have five majors, so Grand Slam might not quite be the right term if she wins all of them. Up next is the Women's British Open at St. Andrews on Aug. 1-4.
The Evian Championship is Sept. 12-15 in France.
Park also became the second player to win the U.S. Women's Open after victories in her previous two tournaments. Mickey Wright did it in 1964.
Kim had what would have qualified as a sensational week if not for Park's dominance, finishing at least three strokes better than anyone not named Inbee.
This was Kim's fourth top-four finish at a U.S. Women's Open, but she's still seeking her first major title.
She was a foot away last year at the Kraft Nabisco, but missed a short putt on No. 18 that would have clinched the championship and went on to lose in a playoff.
Soon-to-be Oklahoma State player Casie Cathrea shot 70 on Sunday to match Shanshan Feng for the best round of the day and finish as the low amateur at 9 over.
Lydia Ko, the 16-year-old New Zealander who won the Canadian Open last August to become the youngest LPGA Tour winner, was next at 11 over.
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