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 to seize command at Valhalla.
Schauffele reached the clubhouse with a three-stroke lead over US compatriot Tony Finau with Northern Ireland's Rory McIlroy and Scotsman Robert MacIntyre third on 5-under.
Schauffele already shared the record-low round in major golf history with a 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.
He set a course record at Valhalla, breaking the 63 fired by Spain's Jose Maria Olazabal in the first round of the 2000 PGA Championship.
Olazabal was among several to fire the prior PGA Championship record low of 63, most recently done by Bubba Watson in 2022 at Southern Hills.
Schauffele, seeking his first major title, has endured 19 top-10 PGA Tour finishes since last winning at the 2022 Scottish Open.
The 30-year-old American has had 12 top-10 major finishes without a victory, including runner-up efforts at the 2018 British Open and 2019 Masters. He was eighth at the Masters last month.
