Pettersson, who led by a shot from American Colt Knost overnight, finished with a two-under-par 69 to end the tournament at Harbour Town on 14 under.
That total put him well clear of Zach Johnson of the United States, with Knost a shot further back in third on eight under.
Johnson went round in 70 on Sunday, while Knost saw his hopes of glory ended by a disappointing 74.
Donald ended a tournament to forget in joint 37th to lose his world number one ranking to Rory McIlroy.
The Englishman, who closed with a second successive level-par 71, needed to finish in the top eight to retain top spot, but ended on two over, 16 shots behind Pettersson.
The Swede got off to a flying start with three birdies in his first five holes, and he could comfortably afford a bogey sixth on the 15th.
Johnson was also three under through five for the round as he looked to challenge Pettersson, but four bogeys and only two more birdies ended his challenge
Knost's hopes were all but ended by a triple-bogey seven on the third as he completed his final round in three over.
He finished ahead of two more Americans, Kevin Stadler and Billy Mayfair, on six under.
Brian Davis was the highest placed Briton in a tie for 13th after a final-round 73 left him three under overall.
Donald went round in level par, bouncing back from a double-bogey six on the first with birdies on the fifth and sixth as he salvaged some respectability.
The win was Pettersson's fifth on the PGA Tour.
He said: "Every win is really special. It's getting harder and harder to win out here.
"But five wins is great. I'm hoping to add to that. I know it's going to take a lot of effort and hard work and dedication, but I hope I can add to it."
On his unusual approach to fitness, Pettersson, one of the larger me on tour, added: "Just because you don't look like an athlete doesn't mean you're not an athlete.
"We're not running marathons here, we're just walking 18 holes. Maybe some of these guys are overdoing it."