Morikawa, who won the 2020 US PGA on just his second major start, held his nerve superbly on the closing stretch at Royal St George's to become the first player to win two different majors at their first attempt.
A closing 66 gave the 24-year-old a winning total of 15 under par and a two-shot victory, with Spieth also returning a 66 and world number one Jon Rahm doing likewise to share third on 11 under with 54-hole leader Louis Oosthuizen.
Oosthuizen has now finished second, second and third in the last three majors, while Spieth's biggest regret was the missed par putt from two feet on the 18th in round three which cost him not only a shot, but also a place in the final group on Sunday.
Tweet of the day
A simple tweet underlines the magnitude of Morikawa's achievement.
Quote of the day
"I walked in and said 'Is there something that I can break?" – Twenty four hours after missing from two feet for par on the 18th hole in round three, Spieth reveals how he felt.
Shot of the day
Louis Oosthuizen again came up short in a major, but produced the shot of the day with his near ace on the 11th.
Round of the day
Three of the top four players shot 66 in the final round, but Morikawa was the only one to do so without dropping a shot.
Statistic of the day
Louis Oosthuizen fans should look away now.
For the first time all week the par-five seventh was the easiest hole instead of the 14th, a total of eight eagles and 41 birdies leading to an average of 4.325.
The 11th was also the toughest hole for the first time instead of the 15th, the 253-yard par three perhaps unsurprisingly conceding just two birdies and leading to 20 bogeys for a scoring average of 3.260.