Weir stumbled to three back-nine bogeys to shoot a 1-over 72, while Singh birdied the 72nd hole to join Weir at 9-under-par 275. Singh's birdie capped a round of 2-under 69.
'It's unbelievable. It was very exciting all day,' said Singh, who won for the second straight week and fourth time in five starts. 'Unfortunately for Mike, he played pretty well and he missed his chances coming in. It was a tough day. The pins were really tucked up and nobody was doing anything except for Mike was just holding his ground.'
Singh and Weir returned to the 18th tee for the first playoff hole. Despite missing the fairway, Weir managed to knock his second shot on the putting surface, as did Singh.
The Fijian two putted for his birdie and was matched as Weir did the same. That sent the duo to the par-4 17th at Glen Abbey Golf Club.
On the 17th, both players missed the green with their approach shots. Weir blasted out of a greenside bunker to 5 feet. Singh bladed a chip shot 6 feet past the cup. Singh missed his par save and tapped in for bogey.
Weir, looking to become the first Canadian winner of this event since Pat Fletcher in 1954, slid his par putt by the lip of the cup and it was back to the 18th tee.
Singh smoked his drive down the fairway at the 18th, while Weir pulled his tee ball into the right rough. Weir, the 2003 Masters winner, pitched his second shot down the fairway.
Singh's second bounced just over the back of the green. Weir, needing to put his third close, hit a wedge over the right edge of the green and into the water.
After taking a drop, Weir's fifth shot came up 4 feet short. Singh, the 2000 Masters champion, pitched on with his third. He calmly rolled his birdie try by the right edge, but tapped in for par and his seventh win of the season.
'For whatever reason I could never get comfortable on the greens, but outside of that, it was a tremendous week, something I'll never forget,' Weir said. 'You know, I'm disappointed not only for myself, but obviously for everybody who was out there supporting me. It was really special.'
Joe Ogilvie fired his third straight round of 2-under 69 to finish alone in third place at 7-under-par 277. Stewart Cink, the top American Ryder Cup finisher, also posted a 69 to share fourth place at 6-under-par 278. He was joined there by Tom Lehman (64), Hunter Mahan (68) and Justin Rose (63).
Singh opened three strokes behind Weir and was steady to start his round as he opened with seven straight pars. The 41-year-old then dropped in back-to-back birdies from the eighth to move to minus-9.
Singh, a 22-time winner in the PGA Tour, survive the par-4 11th with a par. He played that hole in 7 over par for the week. He slipped to a bogey at the 15th, but erased that mistake with a birdie at the closing hole to join Weir at minus-9.
'I had really good chances coming in,' said Singh, who earned $810,000 for the win. 'I had a good opportunity to get up-and-down out of the bunker on 13 and I didn't and I was really frustrated with that. I three-putted 15, but the putt on 18, I needed to knock that in. It was hard to judge the distance coming in on 18, so I always knew if I made that putt on 18, I would have had a slight chance, and that's all I needed.'
Weir, possibly battling nerves being the hometown favorite, stumbled to a double bogey at the par-4 second after his drive came to rest under the lip of a fairway bunker.
He came right back to birdie the third, and birdies at the fifth and sixth moved him to minus-11. Weir left his par putt at the seventh short to drop a shot. He dropped in a 3-foot birdie at the eighth to get back to 11 under.
Weir climbed to 12 under and a three-stroke cushion with a birdie at the 10th. Things went downhill from there as he tripped to bogeys at the 11th and 13th to slide back to 10 under.
A bogey at the 16th dropped Weir into a share of the lead. He had a chance to win the tournament at the last, but his 8-foot birdie try just missed dropping in the cup and it was on to the playoff.
'Obviously I'm very disappointed,' said Weir. 'For whatever reason, I could not find a comfort level on the greens. When I look back on this tournament, when I have time to reflect on it, that's what will stick with me. I felt like if I could have putted halfway decent, it wouldn't have even been a golf tournament.'
Robert Damron fired a 5-under 66 to finish alone in eighth at 5-under- par 279. Mark Hensby and Jesper Parnevik were one stroke further back at 4-under-par 280.