At just 24 years old, Morikawa claimed his second major thanks to a bogey-free round of 66 and finished at 15-under par.
The American had never played in the links position until last week’s Scottish Open, when he finished 71st, the worst end of his professional career.
But the world number four showed no sign of inexperience as he enjoyed the sweltering conditions in a sandwich off England’s south-east coast with temperatures reaching 30 degrees Celsius.
overnight leader louis ostuizenThe fourth-place bogey allowed Morikawa into part of the lead and he never looked back after a two-shot swing in seventh.
Have a dream debut Congratulations Colin Morikawa, Champion Golfer of the Year #TheOpen https://t.co/x1TfIPewYI
— The Open (@TheOpen) 1626629598000
Osthuizen, who finished at 11-under in the part of third with Jon Rahm, found bunkers around the green to drop another shot twice.
Meanwhile, Morikawa drew the first of three consecutive birdie puts and turned at 14 under.
The 2020 US PGA Champion needed his putter to stay warm on the back nine with Spieth on 10, 11 and crucially 15, breathing down his neck.
But by that time, he had moved to 15-under with another long-range putt in par-five 14th.
Three solid innings coming home made Morikawa the first British Open to win since the stunning success of Ben Curtis at Royal St George’s 18 years ago.
Spieth regretted his poor performance in Saturday’s third round, when a bogey in the final two holes saw him start the second day off Morikawa.
The 2017 British Open champion slipped back to seven-under after dropping another two shots at fourth and sixth, which shows he won’t wait long to add to his three majors.
A seventh-placed eagle began Spieth’s challenge and completed the final 12 holes in six-under par with birdies in the ninth, 10th, 13th and 14th to put pressure on Morikawa.
But Morikawa never became the first person to win two Majors on debut in that event.
Rahm also showed why he was a pre-tournament favorite after landing his first major at the US Open last month with a superb 66, which could have been better but with a putter for a better day.
The Spaniard needed an eagle on the seventh to let him go, but ran out of the hole after four consecutive birdies between 13 and 16.
Osthuizen’s second major wait continues, finishing in the top three for the eighth time since his only success at the 2010 British Open in St Andrews.
The pristine circumstances created an opportunity for one of the chasing packs to launch itself into contention.
Four-time Major Champion Brooks Koepka Late Charge for their first British Open reached eight-under with a five-over par round of 65, but was dropped on Saturday for the equivalent of two overs.
“I didn’t do well on Saturday,” Koepka said. “It really doesn’t matter what I finished today. I didn’t have a chance to win and it’s disappointing.”
South Africa’s Dylan Fritelli, who was called into the field as only 11th reserve on Monday, finished fifth at nine-unders.
World number one Dustin Johnson finished eighth at seven under.
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