The 23-year-old American, playing just his second major, shot a 64 for a 13-under-par total and a two-shot win from England veteran Paul Casey (66) and overnight leader Dustin Johnson (68).

Australian Jason Day (66), Matthew Wolff (65), Bryson DeChambeau (66), Tony Finau (66) and Scottie Scheffler (68) were tied fourth at 10 under.

"It's amazing," Morikawa, who will pocket US$1.98 million ($A2.76m) for the win, said after being handed the Wanamaker Trophy.

"It's been a life goal (to win a major), as a little kid, watching all these professionals, and this is always what I've wanted to do."

It was one of the most dramatic finishes to a major championship in recent memory.

A whopping seven players were tied for the lead late in the round before Morikawa chipped in for birdie at the 14th to break the stalemate.

He then smashed a drive at the short 293-yard par-4 16th and drained the seven-foot putt for an eagle to cruise to victory.

RIGHT: Day finished three shots back at 10 under to share fourth place. PHOTO: Jamie Squire/Getty Images.

Day, the 2015 US PGA Champion, gave hope of a late surge with a birdie at the 10th and a 40-foot birdie at the 11th to lift his spirits.

He had a birdie putt lip around the edge of the cup at the 14th hole and fall miraculously in the hole to keep his title hopes alive - but he still came up short in his quest for a second career major.

Morikawa joins an illustrious group of golf greats to have won a major before the age of 24 including Tiger Woods, Jordan Spieth, Jack Nicklaus, Seve Ballesteros and Rory McIlroy.

It is Morikawa's third PGA Tour victory since he made his professional debut in June last year.

RELATED: Inside Morikawa's winning bag

Day's countryman Adam Scott picked up two shots with a final round 68 and he finished the first major of 2020 at a respectable four under for the championship.

Cameron Smith rounded out the Australian contingent and a closing 70 gave the Queenslander an even-par total.

Tiger Woods signed off in style when a 67 improved his tally to one under for the championship

Woods mixed five birdies with two bogeys for his lowest round of the week in just his fourth tournament of 2020, but finished a dozen shots off the pace.