Watson carded a seven-under-par 63, capping a dominant performance with a three-foot birdie at the final hole at TPC River Highlands in the state of Connecticut.

The American finished at 17-under 263 to become the first three-time winner on the PGA Tour this year.

"It's been an amazing ride, my whole career, but this year has been outstanding," Watson said.

Jason Day was the best-placed Australian and finished in a tie for 12th after finishing the tournament with a score of 69 on Sunday.

Casey started the day with a four-shot cushion over his closest rival, American Russell Henley, but a birdie at the first hole was the Englishman's only one the entire day.

He carded 72 to tie for second at 14-under with Americans JB Holmes (67), Stewart Cink (62) and Beau Hossler (66).

Henley finished a further shot back in a tie for sixth.

Watson took a while to reel in Casey, finally doing so with a 10-foot birdie at the 15th hole.

Watson poses with the trophy after winning the Travelers Championship. PHOTO: Tim Bradbury/Getty Images.

"That's when I think I tied him at 16-under, so that's when I really started thinking, OK, we can win this thing," said Watson, who also beat Casey in the same event in 2015 via a playoff.

Sunday's performance all but ensures Watson a spot on the United States team for the Ryder Cup against Europe in Paris in September.

"I didn't think it was possible to get 10 wins. Then I get 10 wins and now I've got two more," he said.

"Now I just need one more in Augusta and that would be great."

Casey described his day as "terribly frustrating" after posting his eighth career runner-up finish on the PGA Tour, to go with two victories.

He hit some poor drives, including at the par-5 13th, which leaked right into a hazard, but his putter saved him for most of the day, until the 16th, where his missed a four-footer.

"I fought my golf swing all day, as you could see coming down the last couple of holes," he said.