Lunke played safe and steady on the first nine holes for a 1-under 35 at Pumpkin Ridge Golf Club. Kelly Robbins made the turn at 1 over, while Angela Stanford was four strokes off the lead at 3 over.
This was the first playoff in the U.S. Women's Open since Se Ri Pak won at Blackwolf Run in 1998, and the first involving three women since Laura Davies beat JoAnne Carner and Ayako Okamoto in 1987.
Lunke, a Stanford alum who has been on the LPGA Tour just two years, amazingly missed the green through her first seven holes. But a 15-foot birdie putt on the par-4 No. 6 put her at 2-under.
She had trouble with the par-5 No. 7, when her shot off the tee went into the deep, wet rough on the Witch Hollow course. Her second shot traveled just 3 feet and she went on to settle for a bogey, her only dropped shot on the front.
Robbins, a 12-year tour veteran seeking her first title of any kind in four years, bogeyed three of the first five holes to fall four strokes back, but narrowed the gap with a pair of birdies.
Stanford, who was coming off her first tour victory in the ShopRite LPGA Classic, opened the playoff round with 2 straight bogeys and she was unable to recover.
The three-way playoff was set up Sunday when all three players came in with 1-under 283s.
Annika Sorenstam needed a birdie on the No. 18 to win, but she hit a 4-wood into the trees, chopped a wedge into the bunker and ended her bid for a second straight major championship with a bogey to finish at even par.
Robbins birdied two of the last three holes Sunday to close with a 2-under 69.
Lunke made six bogeys in a round of 75, but kept her composure and had a chance to win with a 15-foot birdie putt that was right on line but came short.
The surprise was Stanford, who made a slick, downhill birdie putt from 20 feet on the par-5 No. 18 on the Witch Hollow course.