The 34-year-old American fired a bogey-free seven-under par 64 to finish 72 holes on a tournament record 22-under 262 at Harbour Town Golf Links in Hilton Head Island, South Carolina.
It was the second triumph of the year for Simpson, who won at Phoenix in February, and it came after a storm delay of two hours and 47 minutes left players racing to finish before darkness.
“It was a crazy day,” Simpson said. “I’m speechless right now.”
Simpson, set to jump from ninth to fifth in the world rankings, made four birdie putts from beyond 10 feet and two-putted from 41 feet at the par-5 15th in an epic closing run.
“It was a long day on the golf course,” Simpson said. “I didn’t get it going until 12 and then it got going and I felt confident.
“I’m amazed I’m standing here right now.”
Mexico’s Abraham Ancer, seeking his first US PGA crown, was second on 263 after a 65, coming up short on a 37-foot birdie putt at 18 to force a playoff.
“I probably had my best ball-striking week of my career,” Ancer said. “It was awesome.”
American Daniel Berger, who won last week at Colonial, shared third with Britain’s Tyrrell Hatton on 264, with Spain’s Sergio Garcia and Chile’s Joaquin Niemann fifth on 265 and US four-time major winner Brooks Koepka seventh on 266.
The Heritage, whose prior 72-hole record was 264 by Brian Gay in 2009, was rescheduled from a typical April slot as the tour’s second comeback event from a three-month coronavirus pandemic shutdown. It was played without spectators, except for those in course-adjacent backyards.
Simpson sank a 10-foot birdie putt at the par-4 12th and a 22-footer for birdie at 13 to grab a share of the lead at 19-under with Hatton and Ancer.
After tap-ins to par 14 and birdie 15, Simpson shared the lead with Ancer and Berger, who chipped in from 38 feet to birdie the par-3 17th.
Simpson reclaimed the lead alone with a 14-foot birdie putt at 16, then sank an 18-footer at 17 for his fifth birdie in six holes to reach 22-under.
He closed with a par at 18, leaving only Ancer as the final threat in the twilight drama.
– Koepka close –
It was a season-best finish for Koepka, who seeks a third consecutive PGA Championship victory in August.
“I’ve been putting in good work and starting to see the results,” Koepka said. “I feel very close. I just felt good to be back in contention again.”
Hatton, Ancer, Simpson and Ryan Palmer shared the overnight lead at 15-under, with 23 rivals within four strokes.
South African Dylan Frittelli, ranked 116th, sent a signal low scores were available with a career-low, 62, making birdies on six of the last eight holes for the clubhouse edge at 17-under before the 54-hole leaders teed off.
“I was just trying to stay out there and keep playing and get all the good golf out of me, squeeze it out,” said Frittelli. “That was just a nice, consistent round.”