The 23-year-old had eight birdies in a bogey-free round at Taiheiyo Club Minori Course in Omitama to finish with a 20-under 268 for the tournament.
Five of those birdies came on consecutive holes on the back nine, 11 through 15, as she surged into the lead.
Australia's Hannah Green was one of six players to share 12th place on 11-under after a final-round 70.
Sakuma opened the day trailing third-round leader Aki Iwai by three shots, but as the former conquered the back nine, Iwai couldn't keep pace.
After playing the front nine at even par, Iwai's double bogey on the par 3 13th just about negated her three birdies on the back nine, resulting in a 71 on the day.
Iwai finished in second at 16-under, four shots behind Sakuma. It was her only round in the 70s of the week, finishing the first three at 66, 69, 66. For the tournament, Iwai made 23 birdies and had five bogeys and one double bogey.
Sakuma has eight career wins on the LPGA of Japan Tour, four of them this season.
"Actually I could win at the beginning of the season, but it took a while to get a second win," she said.
"That second win was the biggest win for me in my season this year, and that win actually gave my confidence and that led me to win this tournament on US LPGA Tour."
Rookie Erika Hara of Japan was third, her best finish on the LPGA Tour, after her round of 70 left her five shots behind Sakuma.
Arpichaya Yubol of Thailand (72 on Sunday) and Japan's Yuna Araki (70) were six shots back and tied for fourth.
Yubol, 24, was one shot behind Iwai entering the fourth round, and she played in the final group. She said the course was a bigger challenge in than on previous days.
"I do my best already, but it's golf," she said. "Everything is happen. But I think today they put the pin like so hard, really hard, like not easy like every day."
Defending champion Nasa Hataoka fired a 71 to tie for sixth place at 13-under, seven shots out. She held the 36-hole lead after a 64 on Friday.
With her victory, Sakuma became eligible to join the LPGA Tour.
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