By Larry Fine

LANCASTER, Pennsylvania, July 12 (Reuters) - Young South Korean Chun In-gee won the U.S. Women's Open at Lancaster Country Club on Sunday for her first major championship title in an exciting finish over the final holes.

The 20-year-old Chun, a seven-times winner on the LPGA of Korea Tour playing in her first U.S. Open, shot a brilliant four-under-par 66 in the final round to finish on eight-under 272 for a one-stroke victory over compatriot Amy Yang (71).

World number one Park In-bee of South Korea (67) finished tied for third at five under par with Stacy Lewis (70).

Chun began the day four shots behind Yang, but the leader began to falter on the back nine and by the 14th hole there was a three-way tie for the lead at six under par with world number three Lewis joining Chun and Yang.

Chun seized the lead with a nine-foot birdie putt at the 15th hole and the young Korean added birdies on the next two holes to reach nine-under.

Lewis fell out after a double-bogey when she found the water in front of the green at the par-four 15th.

But Yang, who had opened the door with back-to-back bogeys at 14 and 15, eagled the driveable par-four 16th and birdied 17 to move within one stroke.

Chun opened the door a crack by hitting her tee shot at 18 into deep rough. After punching out, she pitched on to the green but missed a curling 12-foot par putt for bogey.

Yang needed par to force a three-hole playoff, but she also found the left rough off the tee and slid her 10-foot par putt by on the left to end her championship bid.

(Editing by Gene Cherry)