American Kupcho closed with a one-under 71 on Sunday to match playing partner, and compatriot, Nelly Korda and Irishwoman Maguire (65) at 18-under 270 at Blythefield Country Club.
"It's really special," Kupcho said.
"I've been really close, but this leaderboard was stacked. I think that's what I'm most proud of, is the players that were up there with me. It was a very close battle to the end and I'm proud of it."
Hannah Green was the pick of the Australians in a tie for 12th at 13 under after a final round 69.
Minjee Lee, the US Women's Open winner, was 11 under after a 74 with Su Oh (72) finishing two strokes further back.
Korda, last year's winner, had a 72 after taking a one-stroke lead over Kupcho into the final round. Korda dropped out of the playoff with a three-putt par on first extra trip down the par-5 18th.
Kupcho won for the second time on the LPGA Tour, following her major victory in early April at Mission Hills in the California desert.
The 25-year-old got off to a slow start with a double bogey on the par-4 third and a bogey on the par-4 sixth. She rebounded with an eagle on the eighth and birdies on the ninth, 12th and 14th.
"I struggled a little bit at the beginning, so it tells me I can really come back from it," Kupcho said.
"I've always known I'm a back-nine player and that definitely came into effect today."
The second-ranked Korda made her second start since returning from a four-month break because of a blood clot in her left arm. She tied for eighth two weeks ago in the US Women's Open in her first event since early February.
"If you told me I think three, four months ago when I was in the ER that I would be here I would being extremely happy," Korda said.
Korda also is the defending champion next week in the KPGA Women's PGA at Congressional.
Lydia Ko was fourth at 17 under after a 68 while Lexi Thompson, the 2015 winner, closed with a 70 to tie for fifth at 16 under.
"Of course, it's not the outcome that I wanted, but I felt strong the whole week and played great golf, and nothing but positives to take into a major next week," Thompson said.
