Scott birdied his 18th hole Sunday at Riviera Country Club to tie Chad Campbell at 9-under-par 133. Campbell had finished his second round on Friday.
The third round had begun Sunday afternoon, but more rain Sunday night into Monday rendered the course unplayable and the tournament was officially shortened to 36 holes.
It is the first time in Nissan Open history that the event was shortened to 36 holes. In the first 78 years of the tournament, only in 1993 had the event played less than the scheduled 72 holes. That year Tom Kite won a five-way playoff after 54 holes.
This was the first weather-shortened event on the PGA Tour since the 2002 Southern Farm Bureau Classic, which was won by Luke Donald. The last PGA event that only went 36 holes was the 1996 Buick Challenge.
In the playoff, Scott teed off first at the par-4 18th and found the left rough while Campbell hit his tee ball right down the middle of the fairway.
Scott's second came to rest short of the green in the fairway. Campbell lost his second shot right of the green and received a free drop from a puddle.
Campbell pitched his third 4 feet past the cup after Scott had chipped to 3 feet. Campbell hit a solid putt, but it slid over the left edge of the cup and rolled 2 feet by.
Scott stepped up and calmly sank his par putt for the win. The win will not be an official PGA Tour title, though, since the event only played 36 holes.
'It's been a tough week. Very unusual week, but I knew I had to make that putt on Sunday and it worked out in the end,' said Scott, who claimed $864,000 for the win. 'A win's a win. It's nice and I seem to be playing well. That was a little match play there leading up to this week.'
The PGA Tour heads to Carlsbad, California this week for the WGC-Accenture Match Play Championship.
Campbell lost in his first playoff on the PGA Tour.
'You know I didn't get to play any golf from Friday afternoon until then, but I just hit a bad second shot there and not a very good putt,' Campbell said. 'That wasn't the easiest putt, a little slider, a little four-footer. I should have made it, but Adam made a good par from where he was.'
Darren Clarke and Brian Davis shared third place at 8-under-par 134. J.L. Lewis and Colin Montgomerie were one stroke further back at minus-7.
Kevin Sutherland, the 2002 WGC-Accenture Match Play Championship winner, led a group in seventh place at 6-under-par 136. He was tied there by Bob Tway, Fred Funk, Robert Allenby, Billy Mayfair and Aaron Baddeley.
Tiger Woods needed to finish solo fourth or better to reclaim the top spot in the World Golf Rankings, but ended in a tie for 13th at minus-5 after double bogeying his final hole.
Mike Weir, the 2003 and 2004 Nissan Open winner, ended the event at 2-under- par 142.