World number two Ko Jin-Young capped a 69 with a curling birdie putt on the 18th hole to capture the Portland Classic, which was shortened to 54 holes for the second straight year due to poor weather.
Ko, who lost her No. 1 ranking to Nelly Korda earlier this year, finished with an 11-under 205 total to win by a dominating four strokes.
She finished off her second win of the season and ninth of her LPGA career with a 25-foot winning putt at the Oregon Golf Club in West Linn, Oregon.
Organizers cancelled the third round after a massive amount of rain fell on the course on Friday.
"I really wanted to play yesterday, but we couldn't play. I tried to make a bogey-free round today, and I made it," said Ko, whose last competition was the Tokyo Olympics in early August.
"I had a lot of missed shots and I had a lot of missed chipping, so I had to make great par saves, but I made it."
This marks back-to-back years where the event was hit by the weather. The 2020 tournament was also shortened to 54 holes because of poor air quality due to wildfires, with Georgia Hall beating Ashleigh Buhai on the second playoff hole.
Ko had a one-shot lead when Saturday's third round was cancelled, but the 26-year-old never wavered in the final round on Sunday.
She went from eight under to 11 under to cruise to victory over Australian Su Oh and fellow South Korean Lee Jeong-Eun.
Her bogey-free round included birdies at Nos. seven, 11 and 18. This was her first tournament on US soil since winning the Volunteers of America Classic in July.
Oh and Lee both earned their best finishes of their LPGA Tour careers with their tie for second at seven-under. Oh had previously finished second three times, while Lee's previous top finish was a tie for third.