Reigning Olympic champion Schauffele started off the 10th tee and birdied five of his first nine holes, then birdied four more after the turn in a bogey-free round for a three-shot lead at rain-softened Valhalla.
"It feels great," Schauffele said. "If someone told me I'd shoot nine-under I'd have taken it. It's a great start to a big tournament."
The American topped the leaderboard over countrymen Tony Finau, Mark Hubbard and Sahith Theegala on 65.
Northern Ireland's Rory McIlroy, South Korean Tom Kim, Scotsman Robert MacIntyre, Belgium's Thomas Detry and Americans Tom Hoge, Collin Morikawa and Maverick McNealy shared fifth on 66.
Top-ranked new dad Scottie Scheffler fired a 67, including a 167-yard, one-bounce eagle from the fairway on the first hole with a 9-iron.
"I tried to hit it right at the pin. I was fortunate to get the distance right. It's nice to get a little bit of luck there," Scheffler said. "I did a lot of good things today."
Thursday's 64 players under-par were the second-most below par after any round in tournament history - and more rain is expected Friday and Saturday.
Schauffele already shared the record-low round in major golf history with his 62 from the first round of last year's US Open at Los Angeles Country Club, matched by American Rickie Fowler in the same round and South African Branden Grace at the 2017 British Open at Royal Birkdale.
"I can't nit-pick. I'll take a 62 in any major any day," Schauffele said.
He set Valhalla's course record, breaking the 63 fired by Spain's Jose Maria Olazabal in the first round of the 2000 PGA Championship.
Asked where it ranked among his greatest rounds, Schauffele said, "Up there. It's only day one. But I'm very content with the shots I played and the putts I rolled."
Schauffele, seeking his first major title, has had 19 top-10 PGA Tour finishes since last winning at the 2022 Scottish Open.
That includes a humbling loss to a McIlroy last Sunday after dominating the Wells Fargo Championship until the final holes.
Schauffele has had 12 top-10 major finishes without a victory, with runner-up finishes at the 2018 British Open and 2019 Masters.
"I've been playing some really solid golf, been having a lot of close calls," Schauffele said. "Me and my team say why not just keep plugging along."
Schauffele birdied the par-3 11th from inside three feet, sank a 13-foot birdie putt at the par-4 13th, another from just outside 16 feet at 15, one from just past seven feet at 16 and a five-footer to birdie the par-5 18th.
On the front side, Schauffele sank a four-foot putt to birdie the second, birdie putts of four feet at the fourth and just inside 10 feet at the fifth, then sank a tap-in birdie at the par-5 seventh.
At the ninth, Schauffele missed a birdie putt of just under 32 feet for a 61.
McIlroy scrambles to 66
McIlroy battled his own personal turmoil in the first round, which came less than 48 hours after his shock divorce filing from wife Erica became public.
The four-time major winner, who last won a major 10 years ago at Valhalla, made six birdies against a lone bogey after a back-nine start.
"A little erratic off the tee," said McIlroy. "But I scrambled well and made some great putts. It wasn't the greatest 66 but 66 it was."
Bogey-free Finau tied his second-lowest major round at 65, only behind his third-round 64 at the 2019 Masters.
"A really nice round," Finau said. "The course was there for the taking if you hit it in the fairway."
Five-time major winner Brooks Koepka, the defending champion, eagled the par-5 seventh and birdied eight to shoot 67.
American Jordan Spieth, a three-time major winner who would complete a career Grand Slam with a victory at Valhalla, opened on 69.
Tiger Woods, a 15-time major champion, fired a 72 with three birdies and four bogeys.
"It was a grind. I didn't drive it particularly well," Woods said.
