Earlier, Thomas (68) and fellow American List (69) finished regulation at eight-under 272, one stroke ahead of Swede Alex Noren (67).
Tiger Woods (70) was eight strokes behind in 12th place.
Thomas, 24, was pleased with the patience and course management he displayed to collect his eighth PGA Tour victory, and his second this season.
"It was another level of difficultly, not only the amount of people I was trying to beat, but this golf course in itself," he said of a tournament that was played mostly in testing winds.
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"I just stayed patient. I thought I stayed very calm all day. I know what this back nine holds and I know how tough it can be. It feels awesome to come out on top."
Thomas, who won his first major last August at the PGA Championship, looks increasingly likely to be one of the favourites at the year's first major, the U.S. Masters at Augusta National in April.
With Sunday's victory, he improved from fourth to third in the world, swapping places with Jordan Spieth.
Dustin Johnson and Jon Rahm remain first and second.
Thomas won on Sunday after a poor drive at the 72nd hole prompted him to lay up, from where he stuck a precise wedge shot to two feet to match List's tap-in birdie.
Noren, meanwhile, missed an 18-foot birdie putt at the last to slip out of the playoff.
Thomas was flawless in the playoff, reaching the back fringe in two shots and then lagging his eagle putt down to three feet.
List has yet to win on the PGA Tour but felt coming so close had energised him for the season.
"This one's going to sting a little bit but I know I've found a restored passion for what I'm doing out here," the 33-year-old said.
"I was proud of the way I fought back after not a great front nine ... I know I'll be back in this position again."
(Reporting by Andrew Both in Adelaide, South Australia; editing by Ken Ferris/Greg Stutchbury)
