Park reached the green on the par-5, 471-yard deciding hole in two shots and two-putted for her second career LPGA victory. Stanford needed three shots to get on the back fringe and her long birdie putt ended up a few feet short.
Both players birdied the final hole of regulation at Grey Silo Golf Course to sit tied at 26-under 258. Stanford closed with a 7-under 64 while Park, who led after the third round, had a 65.
The birdie run continued for the first two playoff holes before Park ended the drama on a hot, hazy afternoon in southwestern Ontario.
Catriona Matthew of Scotland (66) was alone in third place at 23 under. South Korea's Inbee Park (68) finished well back at 16 under, ending her three-tournament winning streak.
Stanford was unlucky on the third playoff hole when her tee shot found the rough and settled into a depression. She used a four-wood to get out but didn't hit it clean, with the ball landing in a fairway bunker.
She did well to get it to the back fringe on the back of the green but with Park already laying two, Stanford needed to chip in to force the issue. When her ball settled a few feet from the hole, Park had the opportunity to go up and down for the victory.
Park's 45-foot uphill eagle putt settled a few feet from the cup and she tapped in for the win. Park earned $195,000 for winning the $1.3-million tournament, while Stanford had a payday of $120,353.
Matthew made a move early in the round and picked up her third straight birdie on No. 7 to move into the top spot with Stanford and Park in the final group, both one shot behind.
The tournament quickly turned into a three-player race.
Matthew birdied No. 11 and 12 but a bogey on the 15th hole snuffed her momentum.
Stanford had seven birdies over a bogey-free round, while Park made her move on the back nine with birdies on five of her last eight holes.
Sitting tied at 25 under entering the final hole of regulation, Stanford reached the green in two and hit a two-putt for birdie. Park pushed her approach shot wide and hit a nice chip from beside the grandstand to within four feet before draining the putt to force a playoff.
The players hopped into golf carts and returned to the tee to play until there was a winner.
Park had a chance for eagle on the first playoff hole, hitting a beautiful approach that bounced once before settling eight feet from the cup. She pushed her eagle putt wide and settled for birdie.
Stanford chipped onto the green from beside the fringe and hit a short putt for a birdie of her own.
On the second playoff hole, both players landed in the first cut beside the green. Stanford went first and chipped it within three feet and Park did the same before both players sank their putts.
Scores were low for all four rounds as players took advantage of wide fairways and greens that remained moist after heavy rain earlier in the week.
"It's pretty crazy that you shoot 15 under and you're not in the top 10, you know," said American Michelle Wie, who closed with a 66. "It's a birdie-fest out there."
American Brittany Lang won the inaugural edition of the event in 2012 but missed the cut this year by one stroke.
Park, 26, earned her only other LPGA victory at the 2011 CME Group Titleholders. Stanford was looking for her sixth win on Tour and first since the 2012 HSBC Women's Champions.
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