The 20-year-old from South Korea made a lengthy birdie putt on the 72nd hole to emerge from a Sunday scramble with an even-par 71 and claim the U.S. Womens Open.
She outlasted playing companion and third-round leader Cristie Kerr, who struggled from the outset in the final round and failed in her bid for a second Womens Open title in three years.
Ji recovered from two bogeys in her first four holes and a double-bogey at the 10th, making three birdies over the final six holes to finish at even-par 284 at Saucon Valley Country Club.
Another of a legion of South Korean players who were inspired to play the game by 1998 champion Se Ri Pak, Ji claimed the biggest prize in women's golf in just her second try. Last year, she tied for 42nd.
She is the second straight South Korean to win the event, following Inbee Park. Countrywoman Birdie Kim claimed the championship in 2005.
Candie Kung of Taiwan had a 2-under 69 and was alone in second at 1-over 285.
Despite her struggles, Kerr held the lead until the back nine and shot a 4-over 75, tying In-Kyung Kim of South Korea for third at 2-over 286.
Ji said the double-bogey at the 10th had a calming effect, and thats when she tried to focus on making a run. She went on to make birdies at 13th, 14th and 18th.
Up until that point, Cristie Kerr was so far ahead, I just didnt think anyone was going to be able to catch her, Ji said. But after that double-bogey on No. 10, I basically cleared my mind and said lets go and play out the rest of the round.
She punctuated her steady back-nine run on the final hole by driving into the center of the fairway, landing her approach about 20 feet from the pin. She steadied her shaking hands and rolled the birdie try into the center of the cup.
Ji said the final putt was about 12 feet, but it looked closer to 20.
Ji, who won the 2008 Wegmans LPGA, pumped her fists and embraced caddie Zac Austin after the winning putt dropped. Kerr gave her playing partner a long embrace.
Kung had continued a charge into contention she started in the third round when she vaulted from 37th to a tie for fifth. She completed her round before Ji and Kerr and was warming up for a possible playoff appeared headed for a playoff after a 2-under 69 left her at 1-over 285.
But Kungs push fell short and she settled for second when Ji rolled in the winner.
I was going out there trying to make par all the way around, Kung said. Par is going to be a good score by the end of the week. Even par won the tournament.
An inch of rain fell on the Old Course overnight, softening the fairways and making the greens more receptive to shots. But a steady breeze helped dry out the putting surfaces and players continued to struggle on the speedy, undulating greens.
Obviously, today wasnt my day, said Kerr, who had 35 putts in the final round. Nothing went in. Even the good putts I hit didnt go in, and thats kind of rough.
You need to get that good feeling and that good momentum on the greens at the Open.
The 2007 Womens Open champ failed to produce anything similar to the three workmanlike rounds that gave her the only under par round through the first three days and a two-stroke lead over Ji heading into the final round.
She had a bogey on the first hole, and had a birdie at the third to get back to 2 under, Kerr had back-to-back bogeys at the fifth and sixth. She dropped into black number for the first time in the championship with a bogey at the 13th.
Her troubles continued down the stretch as she rolled a birdie putt past the hole at the 16th and missed the comebacker, settling for a bogey that dropped her to 2 over. She failed to make lengthy birdie putts at the 17th and 18th.
Not playing the way I did the last three days, this golf course is pretty much all I can handle, and I just didnt play as well, Kerr said.
Brittany Lincicome, who made an eagle putt on the 72nd hole earlier this year to win the Kraft Nabisco, placed fifth after a 1-under 70 left her at 3-over 287.
Top-ranked Lorena Ochoa posted a final round 1-over 72, tying for 26th at 9-over 293 and failing in her bid to claim her first Womens Open.
Duramed Futures Tour player Jean Reynolds was in the next-to-last pairing, poised to make a run. But she stumbled in the final round, shooting 6-over 77 for an 8-over 292.
Jennifer Song was the low amateur, finishing at 7-over 291 after a closing 72.