Watson beat Spieth and Sweden's Jonas Blixt by three shots after a closing 69 at Augusta National, becoming the 17th player to win more than one green jacket.
That list features the likes of Jack Nicklaus, Arnold Palmer, Gary Player, Nick Faldo, Tiger Woods and Phil Mickelson, but Watson was not about to accept the label of an elite player.
"No, no," the 35-year-old left-hander said. "Again, I just got lucky enough to have two green jackets. I'm just trying to keep my tour card every year and if people say that I'm a good player, that's great.
"I'm trying to play golf for a living. I'm not trying to play golf for everybody to tell me how great I am or I'm one of the greats of the game. I play golf because I love it. I love the game, I want to grow the game. The game has brought me everything that I've ever owned in my life."
Watson crucially matched Spieth's birdies on the par-three fourth and sixth - the former after Spieth had holed out from a greenside bunker - but still trailed by two shots after his playing partner also birdied the seventh.
However, Spieth then three-putted the eighth and dropped another shot on the ninth, holes Watson birdied to be out in 33 and claim a two-shot cushion heading into the back nine.
"Eight and nine were really the turning point where momentum kind of went my way," added Watson, who hit a 366-yard drive and sand wedge into the par-five 13th to set up his only birdie on the back nine. "Then the group in front of us and other groups, you could just tell nobody really caught fire. There wasn't too many birdies after number 10 I don't think.
"I don't remember the last few holes, I just remember hanging on, making pars. Somehow I did and walking up 18 was a little easier this time."
Watson, who beat Louis Oosthuizen in a play-off two years ago, added: "This one is a lot different. The first one is almost like I lucked into it, this was a lot of hard work and dedication and I got back the green jacket after giving it away last year."
As defending champion Watson presented the green jacket to 2013 winner Adam Scott; on Sunday the roles were reversed.
"I told Adam we should just keep switching it back and forth," Watson joked.
Spieth, who was looking to become the youngest ever Masters champion at the age of 20, said: "I've worked my whole life to lead in the Masters on Sunday, I had it in my hands and could have gone forward with it and didn't quite make the putts and that's what it came down to.
"I wanted to get into contention and that's what happened but I can take a lot of positives away. My game held up and I feel like I am ready to win a major. It's just a matter of time and maybe a little bit of course knowledge."
Sweden's Jonas Blixt cannot wait for his next shot at major championship glory after finishing second to Bubba Watson in the 78th Masters.
Blixt, who finished fourth in the US PGA Championship last August, carded a closing 71 for his fourth sub-par round of the week at Augusta National, although he was never able to apply enough pressure to Watson.
"I'm kind of lost for words here," said Blixt, who finished alongside 20-year-old American Jordan Spieth on five under par, three behind the winner. "It was a great day. I played decently, hit my driver a lot better, just didn't get my approaches as close as I wanted and didn't give myself enough opportunities to make birdies.
"Overall a decent round and when you shoot under par at Augusta National on a Sunday, you should be pretty happy. Bubba Watson played better, I got beat and he deserves to win. I congratulate him for that but I learned a lot and have a lot more new experiences and can't wait to come back.
"I don't feel like the moment really got to me, but there were a couple of swings there that were a little quick and I just didn't execute it as good as I wanted to."
Europe's last winner at Augusta remains Jose Maria Olazabal in 1999 and although his fellow Spaniard Miguel Angel Jimenez was fourth, the 50-year-old never threatened to become the oldest major champion ever after a front nine of 38 on Sunday.
"I was playing very well, playing very solid all the week," said Jimenez, who will make his Champions Tour debut this week. "Today my putting is a little bit too cold.
"I'm going to play only next week (on the seniors tour). I plan to focus myself for the Ryder Cup and I need to play on the European Tour for that. I would like to help Europe defend the Ryder Cup. I would love to do that."
Lee Westwood finished seventh after also paying for a slow start, the 40-year-old playing the first four holes in three over.
"It's a little bit disappointing but seventh is my best result this year and the work I am doing with my coach Mike Walker is taking me in the right direction," Westwood said. "The Masters probably came a couple of weeks too early."
Rory McIlroy's previous best finish at Augusta was joint 15th in 2011, when he led by four shots going into the final round before collapsing to a closing 80.
But the 24-year-old was again left wondering what might have been if not for a second round of 77, a closing 69 giving him a share of eighth with 56-year-old Bernhard Langer.
"I don't think I've ever played as good tee to green around this course as I have this week. I just need to take some more chances that I've given myself on the greens," McIlroy said.
"I finished even par on the par fives this week and even for the tournament and that's not good enough. You are looking to play them in 10 or 12 under and that would be good enough to win."
From 1860 to 1962 there was not a single left-handed winner of a major golf title.
New Zealander Bob Charles changed that by winning the 1963 Open at Royal Lytham, but then came another gap of nearly 40 years.
How things have changed since then. After Bubba Watson's triumph on Sunday, the Masters title has now gone to a southpaw six times since 2003:
2003: Mike Weir sinks a seven-foot putt on the final green to tie with American Len Mattiace, then wins with a bogey five at the first play-off hole. As well as being the first leftie to win The Masters he becomes the first Canadian to capture any major.
2004: After no fewer than 17 top-10 finishes in majors, Phil Mickelson finally breaks through. A closing 18-foot putt was his fifth birdie in the last seven holes, but he needed every one of them to beat Ernie Els by one.
2006: Mickelson makes it back-to-back major victories at the Masters - after his triumph at the US PGA - this time defeating South African Tim Clark by two after a closing 69. Two months later he was poised to add the US Open, but double-bogeyed the last.
2010: Mickelson's third Masters win and fourth major title is an emotional one, with both his wife and mother having treatment for breast cancer. A memorable shot over water from behind a tree on the 13th leads to a three-shot victory over Lee Westwood.
2012: Watson sees playing partner Louis Oosthuizen make an albatross two on the second hole in the final round, but with four late birdies he draws level with the South African and wins at the second play-off hole after a remarkable escape from the trees.
2014: Watson shares the lead with 20-year-old friend Spieth going into the final round and trails by two after seven holes, but birdies the eighth and ninth and goes on to win by three from Spieth and Jonas Blixt.