Waugh closed with a 7-under 65 to get into the playoff.
Both players birdied the 18th, which was the first extra hole. They moved onto No. 10 where they both parred as Waugh nearly holed her second shot, but her ball spun off the green. She then two-putted from there for par.
On the third extra hole, which was the 18th, Waugh knocked her fairway wood over the green but got up-and-down for birdie. Pak lost her second shot to the par-5 well right. After a free drop, she chipped on and converted her birdie putt.
They two returned to the 18th tee for the fourth extra hole. Waugh pulled her drive left into the water, while Pak's drive ended in the first cut of rough on the right side.
Waugh knocked her third into the front bunker, before blasting to about seven feet. It would not matter though.
'A bad swing. I don't think I was tired, I think it was just a fast swing,' said Waugh of her tee shot on the last playoff hole. 'I really worked hard on that bunker shot. I wanted to get it close, if not make it.'
Pak knocked her second shot over the green. Her chip shot rolled about 15 feet past the cup. She drained the putt for the win, her second victory of the season and 20th of her career.
'Those were some amazing up-and-downs,' said Pak, who earned $202,500 for the victory. 'I impressed myself. I really worked hard for those. My putting saved the day.'
Pak also raised her mark to 4-0 lifetime in playoffs.
'I really like the challenge,' Pak said. 'A lot of pressure makes me play much better. I don't know the reason, but I enjoy it.'
The South Korean began her final round with three straight birdies from the par-4 first. She settled down to par the final six holes on the front side.
'I was pretty happy on the first three holes, making birdie,' said Pak. 'My putting today was much more solid.'
Around the turn, Pak carded her lone bogey of the day at the par-4 10th. She atoned for that mistake in a big way. When Pak rolled in a birdie putt at the par-3 11th, it began a run of four straight birdies. The last of those four birdies gave her a one-shot lead.
Pak maintained her lead with another birdie at the par-3 16th. She birdied the 18th just ahead of Waugh to create the playoff.
'I made the birdie putt on No. 18. That was one of the best putts I've ever had,' said Pak. 'I was hoping for a playoff myself.'
Waugh had an eventful round with just five pars. She birdied the first two holes, but lost her momentum as she bogeyed the third and fourth to fall back to even par for the round.
The Australian ran off three straight birdies from the par-4 fifth to get to minus-12. After three consecutive pars, Waugh birdied the par-3 11th.
However, she dropped a shot at the next. Waugh responded by birdieing the par- 5 13th. Waugh parred Nos. 14 and 15, to set up her late run. She closed with three straight birdies to force the playoff with Pak.
For Waugh, this was her best career finish on the LPGA Tour.
'I think I would be really disappointed if I left the LPGA without a win,' said Waugh. 'I had a great chance to make that step today. The experience I've gained today, should give me other chances to win tournaments.'
Suzann Pettersen, who held the lead after a stellar 7-under 29 on the front-nine, finished alone in third place at 12-under-par 204 after a 5-under 67 Sunday. Meg Mallon ended one shot further back at minus-11.
Sophie Gustafson and Pat Hurst shared fifth place at 10-under-par 206.
Michelle Wie, the 13-year-old amateur from Hawaii, finished at 3-under-par 213 in a tie for 33rd in her second event on the LPGA Tour.