As Scott collapsed down the stretch, like so many others, Spaun emerged triumphant after a disastrous start to his final round to claim a two-shot victory over Scotland's Robert MacIntyre in Pittsburgh.
Runner-up in a play-off to Rory McIlroy at the Players Championship in March, the unheralded Spaun once again showed his class and resilience with a Sunday comeback for the ages.
After starting the day level with Scott and just one shot adrift of American leader Sam Burns, Spaun dropped five strokes in the first six holes to fall five behind.
But he regrouped after a 96-minute weather delay to run down the leaders, closing with a final-round 72 as the fearsome Oakmont Country Club reduced the world's best players to week-day hackers.
The 34-year-old sealed victory in the most stylish and spectacular fashion with a 64-foot birdie putt - the longest-holed putt of the week - at the last hole.
Spaun's winning one-under-par 279 total earned the Californian the Jack Nicklaus gold medal, the US Open Trophy, a $US4.3 million ($A6.6 million) cheque and his maiden major championship, having never previously posted a top-20 finish at any of golf's four biggest events.
Fittingly, Spaun also joined Nicklaus, Ben Hogan, Tom Watson and Jon Rahm to win the US Open with a birdie-birdie finish.
The champion could not have imagined such an outcome after his luckless start included his ball cruelly ricocheting off the flag and off the green on the second hole.
"Yeah, the wheels were falling off early," Spaun said.
"But it wasn't totally my fault. I was getting some unlucky breaks, bounces - a great shot on two that literally was a two-shot swing.
"It probably would have been pretty close to a good birdie look, but I ended up making bogey.
"I was getting very uncomfortable and I did really well the first three days just not caring and not worrying too much about the outcome, and that's what kept me playing well.
"Today I just started caring way too much, but then the rain happened and it was a nice little reset."
MacIntyre's final-round 68 was enough to secure second outright, with Norwegian star Victor Hovland (73) third a further shot back at two over.
Playing in the final group, Scott faded with a final-round 79 to tie for 12th at six over.
Bidding to become the oldest US Open champion since Hale Irwin in 1990, 44-year-old Scott made a nervy start with bogeys on the first and third holes.
Such was the carnage that, despite dropping four shots for his round, Scott was still in a five-way tie for the lead after scrambling for par at the long 12th.
Alas, the Australian's hopes evaporated in a cruel half-hour after the resumption of play due to a flash storm.
From a share of the lead, he bogeyed the 14th after driving into the lip of a bunker.
Then, even more deflatingly, Scott almost drained a long putt to regain a share of the lead on 15, only for the ball to narrowly miss the pin and roll seven feet past.
He missed the return putt and there was no way back for the popular former world No.1.
Third-round leader Burns also suffered a late meltdown after being controversially denied a free drop on the soaked 15th fairway.
Burns was only one stroke behind at that point, but took a double bogey, then dropped more shots in a demoralising round of 78 to ultimately share seventh spot at four over with 2021 champion Jon Rahm and world No.1 Scottie Scheffler.
Finishing with three consecutive birdies in the low round of the day, a three-under 67 in the best of the morning conditions, Rahm dreamed of a miraculous victory from 11 shots back after claiming the clubhouse lead.
In a most extraordinary finale, Rahm held that lead for some four hours before Scheffler joined the Spaniard on the score with an even-par 70.
But Spaun could not be denied.
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