Hur pumped her fist and started laughing after sinking the putt on the par-4 No. 17 hole at Pumpkin Ridge Golf Club north of Portland. Then a friend chased the South Korean around the green, trying to douse her with champagne.
Hur shot a final-round 65, while Pettersen and veteran Michele Redman each shot a 67 to finish at 13-under 203 and set up the playoff.
Hur, who competed on the Futures Tour last season, had played in 13 previous tournaments this season, making the cut six times.
After a foot-long putt for par to finish her round, she quickly exited the course. But rather than retreat somewhere to watch Pettersen and Redman finish their rounds, she hung out in the clubhouse before heading for the putting green.
Asked to explain her calm demeanor, Hur smiled and said: Nothing to lose.
Pettersen and Redman each birdied No. 17 to help set up the playoff.
Pettersen had four straight birdies to surge atop the leaderboard and at one point take a three-stroke lead, but a double bogey on the par-5 No. 15 hurt her.
The native of Norway had not won since 2007, when she won five tour events. She was a runner-up at the Corona Championship earlier this year and had eight top-10 finishes before Sunday.
While ominous clouds and scattered downpours marked the first two rounds, brilliant sunshine and temperatures in the 80s marked the last day. Michelle Wie, Ai Miyazato and Seon Hwa Lee all finished two shots off the leaders at 11-under 205.
Wie finished the final round with a 66. Playing her first full season on the tour, Wie was runner-up in the season-opening tournament in her home state of Hawaii.
She said shes been gaining confidence, boosted by her performance in the Solheim Cup. The United States defeated the European team in the competition last weekend in Sugar Grove, Ill.
Its not really pressure, she said about getting her first victory. I just want it to happen already.
Rookie Anna Nordqvist, who won the LPGA Championship earlier this year, was a one-stroke leader going into Sundays final round. She shot a 72 to finish at 10 under.
Defending champion Cristie Kerr finished well back at 5 under with a 72.
The tournament was moved this year to the Ghost Creek course at Pumpkin Ridge, about a 20-minute drive west of Portland. It had been played the past 19 years at Columbia Edgewater Country Club, near the Portland International Airport.