It was the perfect tune-up for Scheffler who is hot favourite to win his third major at this week’s US Open at Pinehurst.
On a tough day for scoring at Jack Nicklaus’ Muirfield Village, Ohio, Scheffler’s advantage was down to two after nine holes.
But while Hadwin, who had started the strongest with three birdies in his opening seven holes, fell away with five bogeys in his final 11 holes to finish third on four under, Morikawa continued to press.
The lead was down to one when two-time major winner Morikawa rolled in a 30-foot birdie putt on the par-three 12th, while Scheffler missed his attempt from a third of that distance.
The advantage was back to two when Morikawa bogeyed the 16th, but Scheffler dropped a shot on the 17th to leave him with a one-shot lead with one to play.
Both players overhit their approaches to the final green but both then chipped out to around five feet. Scheffler was up first and rolled his effort into the middle of the cup to seal the win and $4m (£3.14m) first prize.
Amid the celebrations Morikawa knocked in his par putt to close with a one-under 71 and seven under total.
Straka, who had been level with Morikawa on six under at the start of the day, fell away with a run of four bogeys in five holes from the 10th, but a chip-in birdie at the last saw him finish with a 76 to end joint fifth on two under.
England’s Matt Fitzpatrick, who won the 2022 US Open, holed six birdies as he shot the joint lowest final round – a three-under 69 – to also finish on two under, alongside Sweden’s Ludvig Aberg (74).
Rory McIlroy closed with a 76 to end two over, one ahead of England’s Tommy Fleetwood who also shot a 76.
Ireland’s Shane Lowry, who was playing with Northern Ireland’s McIlroy, had a disastrous round which included seven bogeys, two doubles and a triple-bogey seven on the 17th as he shot a 13-over-par 85 to finish 12 over.