American Haas, whose great uncle Bob Goalby landed the coveted green jacket in 1968, recovered from a bogey five at the opening hole to set the pace on four-under 68.
Scott, bidding to become only the fourth player to win back-to-back Masters, missed only four fairways but failed to make the most of his birdie opportunities on the greens as he returned a 69 to match his effort at the same stage last year.
Bubba Watson shared second place on 69 along with Scott and the player the American left-hander defeated in a play-off here in 2012, South African Louis Oosthuizen.
Rory McIlroy, the 12-1 pre-tournament favourite along with Australian Scott, launched his campaign with a 71.
World number one Tiger Woods may be missing from this year's line-up following back surgery but there was no shortage of thrills on a sun-kissed day at Augusta National.
The 31-year-old Haas hit the front after bagging three birdies on each nine, including a six-footer at the last that gave him his first three at the 18th at the 17th career attempt.
The pin at the final hole was typical of the tricky flag positions on day one, tucked in on the corner of the putting surface.
"Birdying 18 was a huge bonus," the 2011 FedExCup winner said at the opening Major championship of the year. "I made some nice putts today, a couple of 20-footers, and that can certainly make a difference.
"Today there was a bunch of tough pins and I think sometimes you've just got to say ... you either go at it or you're going to have 50 feet left because it's going to roll on the slopes."
Haas, the son of former US Ryder Cup player Jay senior, refused to get too carried away with his performance.
"I was leading last week after the first round and finished 37th," he said of the Houston Open, "so I know there's tons of golf left.
"I can't expect too much. You've just got to go out there and keep playing golf, try to hit that fairway on number one tomorrow."
Haas was relieved the decision to jettison his brother Jay junior as caddie in favour of the more experienced Scott Gneiser did not rebound on him.
"My brother has been on the bag for a few years and I think I needed a change," said the tournament leader. "I certainly confide in Scott just like I would have with my brother.
"Scott has been under the gun a bunch of times with David Toms and played in Ryder Cups and Presidents Cups. He's seen a lot more pressure-packed situations than most caddies so I certainly feel comfortable with that out there."
Scott felt less than comfortable with the shortest club in the bag, as evidenced by his three-putts for par on the long 13th and 15th.
The world number two was also one of several victims of the notorious Amen Corner stretch - holes 11, 12 and 13.
"At the 12th I received the most incredible ovation I've ever had but I struck my worst shot of the day," said Scott after he dumped his nine-iron into Rae's Creek and took a double-bogey five.
"It was the only weak shot I hit. I think the par-fives are a big key for me here, I didn't take advantage of them and shot 69 so that's a good indication of the quality of my play today."
Argentina's Angel Cabrera, the player beaten by Scott in a play-off last year, had a day to forget as he plunged to a 78 that contained a triple-bogey seven at the 11th.
US Open champion Justin Rose (76), 2007 Masters winner Zach Johnson (78), WGC-Match Play champion Jason Day (75) and US PGA winner Jason Dufner (80) also struggled.
Former world number one Luke Donald was unable to turn around his recent poor form, slumping to a 79 highlighted by a quadruple-bogey eight at the ninth that featured a two-shot penalty for grounding his club in a bunker.
Triple Masters winner Phil Mickelson veered from the sublime to the ridiculous as he ballooned to a 76, matching his previous worst opening round at the championship in 1997 and 2007.
The left-handed American suffered two sevens in his round but also curled in a snaking 60-foot putt for an unlikely birdie three at the 10th.
- Sports & Recreation