Park beat Brittany Lincicome with a par on the first hole of a playoff to end the United States' major streak at three.
"I didn't feel that nervous at all today," Park said. "But once I got to the tee on the playoff hole, I just felt the nerves right away. It was like a replay of last year and experience definitely helped me out. I think I was able to stay calm."
Lincicome was poised to win her second major and keep that American streak alive, but her nerves got the best of her after she led all day.
"Not being in this position for a while, I think it all caught up with me," Lincicome said. "Being second at a major is always a good thing. I feel like I played really, really well this week. If I keep playing the way I did, my time's coming soon. It was nice to be in contention again."
On the playoff hole on Monroe Golf Club's par-4 18th, Park hit her second shot into the rough behind the hole. Lincicome hit her approach to the left fringe, nearly identical to her position on the final hole of regulation when she made a bogey to fall into the playoff.
Lincicome chipped 6 feet past the hole and failed to convert. Park, the winner last year at Locust Hill Country Club in a playoff with Catriona Matthew that took three holes, chipped to 3 feet and calmly sank her par putt for her fifth major title and fourth in the last two seasons.
"Inbee is so darn good. It was so close," Lincicome said. "I need to learn how to control the nerves a little bit more."
Park finished with a 2-under 70 to match Lincicome at 11-under 276. Lincicome had a 71.
Americans had won the first three majors of the LPGA Tour season for the first time since 1999. Lexi Thompson began the run at Kraft Nabisco, Michelle Wie won the U.S. Women's Open and Mo Martin the Women's British Open.
The 26-year-old Park, from South Korea, was coming off a playoff loss to Mirim Lee last week in Michigan. Park also won this season in Canada and has 11 LPGA Tour victories.
"I've been in many playoffs," said Park, who joined Nancy Lopez and Patty Sheehan as the only players to win twice in a row at Rochester. "It's not something I look forward to doing because there's so much pressure. I feel lucky I got an opportunity today."
Park is projected to jump from third to second in the world, passing 17-year-old Lydia Ko of New Zealand. Ko, trying to become the youngest major winner in LPGA history, shot a 70 to finish third at 8 under.
Spain's Azahara Munoz (70) and Sweden's Anna Nordqvist (71) tied for fourth at 6 under.
Lincicome squandered the lead on the final hole of regulation. She hit her second shot to the left fringe and was in a good spot, but a long delay for a ruling on a shot by Suzann Pettersen only heightened the tension, and it showed.
With top-ranked Stacy Lewis among the gallery clapping, Lincicome left her first putt 8 feet short and failed to make par, forcing the playoff.
"I was really nervous coming down the stretch. I was shaking like a leaf," Lincicome said. "It's hard to do anything when you're shaking."
Pettersen, a two-time major winner, started the day a shot behind as she chased her first win this year. But her day went badly at the start and she shot 4-over 76. She tied for sixth at 5 under with Lewis, Julieta Granada, Shanshan Feng and Lee.
Park's clutch birdie putt at No. 17 put her in position to challenge and her par save at 18 was crucial. Her approach on the closing hole landed in the rough to the right of the green and she botched her shot out. Her 12-foot putt left no margin for error and the crowd roared when it rolled in.
Lincicome had held the 54-hole lead at a major only once before, at the 2006 U.S. Women's Open, but she faltered with a closing 78 and finished seventh. This time, she shook off the nerves until the end as the chance to win her second major ended in disappointment. Lincicome won the 2009 Kraft Nabisco.
The tour made the switch this year to Monroe after 37 years at nearby Locust Hill. The Donald Ross-designed course is about 300 yards longer at 6,717 yards and does not have a single water hazard, but it does feature 106 bunkers, more than double the number at Locust Hill, and the wider fairways favor long hitters just not enough in the end.
Tournament officials estimated 25,000 people attended the final round and 100,000 for the four days.