On a great day for the Australian contingent, Marc Leishman is a further shot back in a tie for fifth, while John Senden is at even par.
American Haas leads at the end of a round at a major for the first time thanks to his four-under 68 at Augusta.
Haas, 31, is sure to have plenty of competition as the tournament progresses, with three players tied for second a shot behind him.
Among them is Australian Scott, whose only blemish was a costly one as he double-bogeyed the par-three 12th hole.
Adam Scott of Australia watches his tee shot on the 12th hole. Photo by Rob Carr/Getty Images
South African Louis Oosthuizen and 2012 winner Bubba Watson are alongside Scott after also carding 68s.
Haas made a slow start – bogeying the first – but he followed that up with three birdies on the front nine.
Back-to-back birdies on 13 and 14 had him well set up and he finished with a birdie and bogey for a decent round.
Haas is aiming to better the efforts of his dad, Jay, who managed 16 top-10 finishes at majors without winning.
Scott, looking to claim the world number one ranking, was a good chance to be leading at round's end.
He was four under through 11 holes, starting without a bogey in a dream start to his title defence in Georgia.
But a five at the 12th was a huge blow as he found water at Rae's Creek off the tee.
Adam Scott of Australia looks at his putter. Photo by Andrew Redington/Getty Images
Scott missed a six-foot bogey putt and had to settle for a double, before another birdie at the 14th helped him partly recover.
He is the first defending Masters champion to start his round in the 60s since Fijian Vijay Singh (2001).
Oosthuizen's mixed round included six birdies and three bogeys, while Watson produced the only bogey-free round of the day despite the good conditions.
A group of seven players – Americans Kevin Stadler, Gary Woodland, Jimmy Walker and Brandt Snedeker, Swede Jonas Blixt, South Korean KJ Choi and Leishman – are tied for fifth at two under.
Former champion Fred Couples, 54, managed a one-under 71 to be in contention.
Fred Couples of the United States walks up the second fairway alongside caddie Mark Chaney during the first round. Photo by Harry How/Getty Images
Northern Irishman Rory McIlroy is alongside him, as is 20-year-old American Jordan Spieth and Rickie Fowler.
Englishman Lee Westwood, South African Charl Schwartzel and American Matt Kuchar all carded 73s to be in the mix.
Americans Hunter Mahan and Webb Simpson, Northern Ireland's Darren Clarke and Spaniard Sergio Garcia are a further shot back.
Jason Day has plenty of work to do after he holed three bogeys and a double bogey in a costly four-hole patch to be at three over.
Jason Day of Australia hits an approach shot on the fifth hole. Photo by David Cannon/Getty Images
Last week's Houston Open winner Matt Jones and Steven Bowditch both carded 74s, while rookie Oliver Goss had a four-over 76.
Phil Mickelson will need quite the recovery to win a fourth Masters crown after opening with a 76, along with England's Justin Rose.
Argentine Angel Cabrera, runner-up to Scott in a play-off last year, carded a six-over 78.