Spieth could only watch from the side of the green.
He dazzled the massive crowd early by holing out for birdie from the front bunker on No. 4, and making back-to-back birdies to build a two-shot lead through seven holes. Bidding to become the first player in 35 years to win a green jacket in his first try, Spieth looked to be well on his way.
But he three-putted for bogey on No. 8 — the first 6 on his card all week — as Watson got up-and-down for birdie to tie for the lead. Spieth then made a rookie mistake, leaving his approach below the flagstick on No. 9 and watching it roll back into the fairway, setting up another bogey and two-shot swing.
Whatever prayer he had might have ended at Amen Corner.
His tee shot on No. 12 found Rae's Creek. He missed a short birdie attempt on the 13th.
Watson was too powerful, too experienced, too tough to beat. Spieth closed with six pars for a 72 and tied for second with Blixt, who never went away but never really threatened. Blixt shot a 71.
"Obviously, I've worked my whole life to lead Augusta on Sunday. And although I feel like it's very early in my career, and I'll have more chances, it's a stinger," Spieth said.
Watson finished at 8-under 280 and goes to a career-best No. 4 in the world.
Jimenez, the ageless wonder from Spain, shot 71 and finished alone in fourth. Matt Kuchar lost a share of the lead with a four-putt double bogey on the fourth hole and never challenged again. He closed with a 74 and tied for fifth with Rickie Fowler (73).
Nine players were separated by three shots at the start of the final round only for this to turn into a two-man show. For the opening two hours, it was anything but dull.
After trading pars on the opening hole, either Watson or Spieth — sometimes both — made birdie or bogey over the next nine holes.
Spieth holed out from a bunker for birdie on the tough par-3 fourth. He made back-to-back birdies with a 12-foot putt on the seventh for a two-shot lead.
Two holes to close out the back nine changed everything. Amen Corner swung the Masters in Watson's favor for good.
Watson won for the second time this year, and his second major puts him at the top of the Ryder Cup standings.
He was guided all week by a simple game plan of hitting fairways and greens, and he was calmed by knowing that regardless of how it turned out, he still had a green jacket. Now he has two of them.