Ko, one of the game's best putters, poured in a seven-footer almost identical to one she missed on the third extra trip to the par-five 18th, to win her fourth title of the year and 14th of the precocious 19-year-old's career.
Ko missed a 5-foot birdie putt on the third extra hole, sending the trio back to the 18th tee.
It is her 14th career LPGA title and fourth of the year.
It also continues a staggering run for Ko in which she has finished in the top five in her last five tournaments, including two victories. I said, "hey, deja vu".
The 19-year-old won the LPGA's Marathon Classic in OH on Monday morning, needing all of her trademark composure to survive a four-hole play-off.
That left her tied for second with American Alison Lee, who notched her second straight 66. Even today I had a few that were so close to going in and didn't drop, I felt like I did leave a few out there. "It was good." Birdies at 14 and 17 saw her build a comfortable cushion heading into Sunday's final round on 15-under par 198.
"In this wind, I feel like my score is a pretty solid one".
Stacy Lewis and Ariya Jutanugarn are four shots back in a tie for fourth at 11-under.
All three players had mouth-watering chances to claim victory but grazed edges, misread lines or mis-hit their potential play-off-ending putts. Thats kind of been my motto the last week or so, just believe in it and trust it, and thats what I keep trying to do..
Jutanugarn birdied the final three holes for a 66.
Kim Kaufman entered the final round of the Marathon Classic in a six-way tie for 13th at 6-under. The Australian secured a two-year exemption and a spot in the PGA Championship, but not the Masters.
The 49-year-old Toms was 4 under for the day with four holes left. The American, who has been struggling with a shoulder injury, registered three birdies in the last four holes to jump up the leaderboard.