Reed became the youngest winner of a World Golf Championship, his third win in his last 14 tournaments.
Woods, only three shots behind going into the final round in his best chance this year to win a tournament, said his back flared up after an awkward shot out of the bunker on the sixth hole. He failed to make a birdie in the final round for the first time in his PGA Tour career, and his 78 was his worst Sunday score ever.
Reed is expected to go to No. 20 in the world ranking. In his own ranking, he feels he belongs in the top five.
He cited an amateur career that includes going 6-0 in matches to lead Augusta State to two NCAA titles, followed by three PGA Tour wins in seven months.
"I don't see a lot of guys that have done that besides Tiger Woods and the legends of the game," Reed said. "I believe in myself, especially with how hard I've worked. I'm one of the top five players in the world. I feel like I've proven myself."
He joined some exclusive company. Since 1990, only Woods, Phil Mickelson, Rory McIlroy and Sergio Garcia had three PGA Tour wins before turning 24.
Reed has had a share of the lead going into the final round of all three of his victories the Wyndham Championship last August, the Humana Challenge in January and a World Golf Championship at Doral that featured the strongest field so far this year.
His last two wins were wire-to-wire, including ties.
Watson, who won at Riviera three weeks ago, went bogey-free over the final 27 holes, a strong performance on the overhauled Trump National Doral. He closed with a 68, finishing with par after blasting through the palms, into the grandstand and back into the rough.
Donaldson wasn't so fortunate. He hit into a foot on the 17th for his third birdie on the back nine to get within one shot of the lead. From the 18th fairway after watching Miguel Angel Jimenez go through the green and into the water he blocked his approach away from the flag and into the back bunker. Donaldson blasted out to just inside 15 feet and missed the par putt. He closed with a 70.
Reed finished at 4-under 284.
Puerto Rico Open
Chesson Hadley won the Puerto Rico Open for his first PGA Tour victory, holding off Danny Lee by two strokes in wind gusting to 25 mph in Rio Grande. Making his 13th PGA Tour start, the 26-year-old Hadley birdied the final two holes at Trump International for a 5-under 67. He finished at 21-under 267.