Morikawa emerged from a seven-way tie for the lead in the closing stages at Harding Park by chipping in for a birdie on the 14th hole and then producing a stunning eagle on the 16th, driving the green on the 294-yard par four and holing from seven feet.
The 23-year-old carded a closing 64 – equalling the lowest round of the week – to finish 13 under par and two shots ahead of overnight leader Dustin Johnson and England's Paul Casey, who barely put a foot wrong in pursuit of his first major title at the 64th attempt.
Casey carded five birdies and a solitary bogey in a final round of 66 to record his best finish in a major, with Matthew Wolff, Jason Day, Bryson DeChambeau, Tony Finau and Scottie Scheffler all sharing fourth place on 10 under.
Tweet of the day
Morikawa was flawless on the course but his trophy handling needs a little practice.
Quote of the day
"If he's trying to play mind games, he's trying to do it to the wrong person" – Rory McIlroy feels Brooks Koepka's is wasting his time trying to get into Dustin Johnson's head about the latter's relative lack of major titles.
Shot of the day
Having just been caught by Casey's birdie on the 16th, Morikawa went one better with a stunning eagle from seven feet on the 294-yard par four.
Round of the day
Morikawa had to work hard to par the first two holes but was almost flawless after that with four birdies and an eagle.
Statistic of the day
Anyone looking for next year's winner of the US PGA should perhaps focus on the young guns.
On the up
Collin Morikawa, whose brilliant victory lifted him to fifth in the world rankings after little over a year as a professional.
On the slide
McIlroy's respect for Koepka after the latter's shot at Johnson's relative lack of major success following the third round.