The 27-year-old New Zealander shot a solid closing 68 to triumph at 13-under-par 275 at Sentosa Golf Club on Sunday as Japan's Ayaka Furue (68) and Thailand's world No.2 Jeeno Thitikul (70) tied for second.
The 2024 Olympic gold medallist and Women's British Open champion overcame a middling start before finding her rhythm with a birdie on the par-5 sixth.
A former teen prodigy who claimed the world No.1 ranking at age 17, Ko added back-to-back birdies on Nos. 7 and 8, seizing control of the leaderboard from then on as she secured her 23rd LPGA Tour title.
"I dreamt last night that I won but then I woke up, and I was like, dang, it's not real yet," said Ko. "But I just wanted to focus on my game."
"I started off really steady, and you know, didn't really put myself in trouble that much. I think that was going to be the key for today."
Australia's defending champion Hannah Green had to settle for a share of seventh place as her closing 71 left her seven shots adrift of Ko.
Minjee Lee (73) was a shot further back while fellow Aussie Steph Kyriacou tied for 48th after a 72.
Ko's previous best finish in 11 appearances in the tournament was a second placing in 2015.
"It's exciting to finally add Asia's major to my major collection," she said.
Charley Hull, just one shot behind Ko entering the final round, saw her title hopes fade after a frustrating 74, settling for a share of fourth at seven under alongside Im Jin Hee, who fired the day's best round of 67, and Gaby Lopez (70).
The Singapore tournament is the second of three events on the LPGA's first Asian swing of the year. The final event will be played next week at Hainan Island, China.
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