World number three Day reeled off eight birdies in a bogey-free final round 62 for a 19-under-par 261 total at Plainfield Country Club for his third victory in his last four starts.
A distant second on 13-under was Sweden’s Henrik Stenson, who posted a 66, while two-times Masters winner Bubba Watson (69) finished third, eight strokes off the pace.
Day’s dominating victory lifted the Australian into the conversation of top golfer of the year, particularly after Masters and U.S. Open winner Jordan Spieth relinquished the top ranking to Rory McIlroy when he missed The Barclays cut.
“I’m just trying to improve each and every week but it’s been a special summer for me and it’s not over,” Day told CBS in a greenside interview after rolling in his final birdie putt at the 18th.
“To be able to play the way I did over the weekend here was fantastic. Today was just phenomenal golf. I’m ecstatic and looking forward to next week,” added Day, who fired a 63 on Saturday to share the third-round lead.
Since missing cuts at the Players Championship and the Memorial Tournament in June, Day has been a force on the course.
The 27-year-old tied for ninth at the U.S. Open despite suffering from a vertigo attack that caused him to collapse on the final hole of the second round at Chambers Bay.
Day finished tied for fourth at the British Open, one shot from joining a playoff at St. Andrews won by Johnson.
He bounced back the next week to win the Canadian Open, finished tied for 12th at the World Golf Championships-Bridgestone Invitational and then claimed his maiden major at the PGA Championship at Whistling Straits.
“I’m really going to try and make this push for the FedExCup,” said Day, who leapfrogged Spieth into first place in the four-event series that rewards the ultimate points leader with a $10 million bonus after the Tour Championship in Atlanta.
“It’s been an amazing year for me,” said Day, whose four PGA Tour wins this year also includes February’s Farmers Insurance Open.
“Just the work and dedication I’ve put in over the years, it feels like it’s finally paying off. It’s been a whirlwind kind of summer.”
Day is only one victory short of matching the record wins in one season on the PGA Tour by an Australian.
Jim Ferrier won five times in 1951.
(Editing by Andrew Both)