A commanding six strokes ahead overnight, the world number six carded a bogey-free two-under 70 on a picture-perfect afternoon of unbroken sunshine on the Monterey Peninsula.
“It’s unbelievable,” Spieth, 23, said after becoming the first player since Tiger Woods to win nine times on the PGA Tour before the age of 24.
“Today I hit 17 greens, it was kind of a dream round for ball-striking when you are leading by a bunch and I finally got one (a birdie putt) to go on 17.
“Boring golf,” double major champion Spieth said with a smile after totalling 33 putts on Sunday compared to 23 the previous day.
“I apologise but that was kind of the game plan. That’s what was needed today and fortunately, that’s what we did to close it out.
“We could afford to make a couple of mistakes if I needed to ... fortunately, we didn’t and we just stayed ahead.”
The Texan finished at 19-under 268, while fellow American Kelly Kraft, the 2011 US amateur champion, fired a 67 for second place at 15-under, with compatriot American Dustin Johnson (68) a further stroke back.
Spieth, who won the Australian Open in November, has finished no worse than ninth in any of his past six starts worldwide.
He had produced a putting masterclass that yielded eight birdies at Pebble Beach on Saturday but his putter cooled down on Sunday as he recorded just two birdies at the same venue despite playing rock-solid golf from tee to green.
A two-putt birdie on the second hole was followed by a string of 14 consecutive pars that kept his closest challengers at bay before he picked up another at the par-three 17th, sinking a curling 30-footer before hoisting his putter skywards in celebration.
Australian world number one Jason Day, rebounding from a disappointing 75 in the third round, chipped in for eagle at the par-five 14th on the way to a 67 and a three-way tie for at fifth at 12-under.
Shane Lowry picked up two shots in his final round to leave him on -7 for the tournament and in a share of 14th place.
Seamus Power finished 39th on four under par.