The 23-year-old American, who began the day two shots back of overnight leader Dustin Johnson, shot a bogey-free six-under-par 64 to reach 13 under for the tournament.
Collin Morikawa has claimed his first major championship, delivering an magical eagle on the 16th hole in a bogey-free round of 64 at the 2020 PGA Championship.
The 23-year-old American, who turned pro just 14 months ago, did the unthinkable by winning on Sunday in just his second career major appearance.
Morikawa, who is ranked 12th in the world, eventually finished on Sunday at 13-under 267 at TPC Harding Park in San Francisco.
"It's amazing. It's been a life goal, obviously as a little kid, kind of watching everyone grow up, all these professionals, and this is always what I've wanted to do," Morikawa said.
"I felt very comfortable from the start.
"As an amateur, junior golfer, turning professional last year, but to finally close it off and come out here in San Francisco, pretty much my second home where I spent the last four years, is pretty special," added Morikawa, who grew up in Los Angeles but went to college at Berkeley in San Francisco.
"It was going to take a very, very good round today, and I knew with the leaderboard the way it was looking and everyone out there, you just had to play well."
Johnson settles for a tie
Third round leader Dustin Johnson had to settle for a tie for second with England's Paul Casey.
Johnson shot a 68 and Casey closed with a 66 to reach 11-under 269.
Jason Day, Matthew Wolff, Bryson DeChambeau, Tony Finau and Scottie Scheffler all finished in a tie for fourth, three shots adrift of Morikawa.
Heading into the final round there were ten players within three shots of the lead. The packed leaderboard continued throughout Sunday's round as it looked like any of a half a dozen players could emerge victorious.
When Johnson made the turn after his ninth hole he was tied with three others for the lead at 10 under.
They were soon joined by two others making a six-way tie on the back nine.
Morikawa had four birdies, including two in his first four holes as he got off to a hot start. But it was his eagle at 16 that separated him from the tight pack at the top of the leaderboard.
Morikawa put it all together in the fourth round, keeping pace with the leaders throughout then emerging from the pack with his eagle on 16.
He hit a 293-yard tee shot onto the green, landing it just seven feet from the flag stick. Morikawa made the putt to take the lead for the first time in the round and kept it the rest of the way.
Johnson birdied his final hole, while Casey, who was trying to become the first Brit to win the PGA in almost 100 years, also made par there.
Brooks Koepka failed in his bid for a historic three peat. The two-time defending PGA champion got off to a woeful start with four bogeys on the front nine en route to a four-over 74.
Tiger Woods rebounded from a frustrating third round to shoot a 67 and finish one under for the tournament.
Woods struggled on the greens early in the tournament, but fo und his putting groove Sunday, needing just 21 putts in the fourth round.
China No. 1 Li Haotong, who this week became the first player from China to lead in a major championship, shot a 69 for a six-under 274 total.
Li had another solid round going on Sunday but finished with a double bogey on 18.
Former world No. 1 Rory McIlroy shot a closing 68 to reach two-under 278, 11 shots back of Morikawa.
McIlroy, of Northern Ireland said players who had the most success were the ones who drove the ball straight on the fairways.
"If you missed fairways you were doing well to make par," he said.
It is the first major since the season was placed on hold due to the Covid-19 outbreak. Because of the coronavirus restrictions the 2020 PGA took place with no fans, which was a first.
Players, caddies and officials were tested for the coronavirus before entering the grounds. The list of players who had previously tested positive for the virus included Cameron Champ who was in contention Sunday.
Champ, who was given the all clear before the tournament, shot a 70 in the fourth round to finish at eight under.