After starting the final round in a six-way tie for second, two shots off the pace, Rai closed with a steely and at-times spectacular five-under-par 65 to post a landmark three-stroke triumph over Spanish star Jon Rahm and American third-round leader Alex Smalley.
The 31-year-old world No.44 is the first Englishman to hoist the Wanamaker Trophy since Jim Barnes won for a second time way back in 1919.
And Rai saluted in style, rallying back from three shots behind before making eagle at the par-5 ninth and all but sealing victory with a magical 68-foot birdie putt on the penultimate hole.
"Very surreal," Rai said after becoming the first non-American winner of the PGA Championship since Australian Jason Day in 2015.
"It's been a bit of a frustrating season so to be standing here is definitely outside of my wildest imagination. It's phenomenal."
Rai had jostled for the lead all day as Rahm and fellow heavyweights Rory McIlroy, Xander Schauffele and Australia's resurgent former British Open champion Cameron Smith all duelled down the stretch.
He burst clear of the pack with five back-nine birdies, leaving Rahm (68) and Smalley (70) to share the runner-up spoils.
Rai's incredible birdie on the 71st hole came from almost the identical spot on the green from where Smith's challenge died with a three putt.
"I definitely wasn't trying to hole that putt," Rai said.
"The shadow of the pin gave a really nice line for probably the last 10 feet, so that definitely helped with the visual of the putt.
"But it was so long that I was just trying to put good speed on it, make a good putt, and it just tracked extremely well in the last half, so amazing to see that one go in."
After sitting anxiously on the clubhouse lead for more than four hours, two-time PGA Championship winner Justin Thomas had to settle for a joint fourth, four shots behind Rai, following a blazing final-round 65.
Swede Ludvig Aberg and German Matti Schmid both closed with 69s to share fourth with Thomas.
After missing the cut at six majors in a row, Smith emerged from the golfing wilderness with a tie for seventh at four under alongside McIlroy (69) and Schauffele (69).
The 2022 British Open champion had a series of chances to grab a share of the lead but couldn't find a birdie on the back nine en route to a closing 68.
Rai's maiden major victory comes after just one previous win on the PGA Tour, at the 2024 Wyndham Championship. Â
But the breakthrough had been coming, with Rai making the cut in his previous eight straight majors having featured on the weekend only once before that run.
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