Harold Varner III has won the Saudi International by sinking a near 30 metre eagle putt while Australian Cameron Smith had to settle for fourth place.

Cameron Smith has been left disappointed at his fourth place finish in the Saudi International - but American Harold Varner III was left celebrating the most extraordinary of victories.

Varner, who hadn't won an event in five years, sank an extraordinary near 30-metre putt off the fringe of the final green to make eagle and snatch a one-stroke victory from a disbelieving Bubba Watson.

It was an amazing finale with the 31-year-old American barely able to believe that he'd sunk the putt when he would have been happy just to get down in two at the Royal Greens Golf & Country Club to force a playoff with his compatriot.

Watson had finished his brilliant final round of six-under 64 with an eagle himself to set the clubhouse lead.

However, a 69 from Varner got him to 13-under 269 and enough to bring the former Australian PGA champion the biggest win of his career and a $US900,000 ($A1.3 million) winner's cheque.

For Smith, his last-day push for victory amid the desert near Jeddah followed the path of much of his week -- excellent approach play only to be let down, unusually, by his putting in the windswept conditions on the banks of the Red Sea.

He recorded a one-under 69 to finish on nine under in joint-fourth, one shot adrift of third-placed Spaniard Adri Arnaus (71).

"I played really solid, just felt like I struggled on the greens all week," Smith, who won the opening event of the US PGA Tour season in Hawaii last month, said.

"I was hitting good putts but I just couldn't get them to drop this week.

"It's disappointing - I came pretty close but that's golf.

"I loved the experience here, the course is great but it's really tough when the wind gets up."

Like everyone else, Smith was left astonished by Varner's incredible winning putt, which was followed by the overjoyed American pumping his arms in delight and then having his caddie leap into his arms.

"There's been times where it just didn't go my way and today it did," Varner said.

"Worst-case scenario, we'll go to a playoff and I'd get him there.

"And then it went in, and emotions came out! I love that!"

Watson took it all with good grace. He ran over from where he'd been preparing for the playoff to congratulate his friend.

"I'm not mad at him for beating me -- I'm happy for him," Watson said.

"He's a dear friend of mine, and I applaud him. I love seeing that."

Other Australians to enjoy a decent pay day in the lucrative Asian Tour event were Brad Kennedy, who shot a 68 to finish joint-14th on six under and Wade Ormsby, whose 73 left him at four under in equal 18th.

While Smith will return to play in the US, he said he would like another shot at glory in the Middle East.

"I guess I know my game is in the right spot. I'll have a week off and recharge at home and get ready for (the Genesis Invitational PGA Tour event in) LA," he said.

"I'll see what happens with the schedule and hopefully we can be back soon."