The world number four went into the final round holding a slender one-shot advantage over playing partner and two-time champion Phil Mickelson.
With seven holes remaining for the final pair it was Mickelson and KJ Choi who shared the lead on 12 under, but Westwood was a shot behind alongside Anthony Kim, with Tiger Woods also in the hunt despite a dreadful start.
Westwood twice showed his character in bouncing back from bogeys early on.
A wayward drive resulted in a dropped shot at the first, but he responded by reaching the par five second in two and recording a birdie to move back to 12 under.
Another bogey at the short fourth saw Westwood drop back alongside Mickelson, but the Englishman regained his advantage once more with a birdie on the fifth.
Westwood then pulled off a spectacular save on the par four seventh, almost holing his bunker shot for a three when he appeared likely to drop a shot.
Mickelson, by contrast, started with seven straight pars before claiming a birdie at the eighth courtesy of a sublime wedge.
Westwood three putted the ninth, but Mickelson got up and down to remain on 12 under, before repeating the feat on the tenth, where a wayward drive looked set to prove costly.
While much of the focus was on the final pair, Choi was giving himself an excellent chance of claiming a first Major victory with a nerveless final day performance.
Playing alongside Woods for the fourth round in succession, he birdied the second before rescuing an unlikely par on the third with a stuning putt from off the front of the green.
Further birdies followed on the sixth and eighth and the Korean eventually moved into a share of the lead when another excellent approach shot set up a birdie at ten.
Kim was another to come from under the radar on Sunday afternoon, with birdies at 13 and 14 lifting him to nine under.
The American then fired a sensational approach to the 15th and held the subsequent eagle putt to move alongside Westwood.
Woods looked to have dropped out of contention when he followed an opening bogey with further dropped shots at the fourth and fifth.
However, there was some cheer for the world number one when his second shot to the seventh spun across the green and into the hole for an eagle two.
The four-time Masters champion gained further strokes at eight and nine to resurrect his hopes of claiming yet another Green Jacket, although his revival was halted by a bogey at 11.
Fred Couples matched Choi's bogey-free front nine of 33, but his hopes of victory suffered a seemingly fatal blow around Amen Corner.
After dropping a shot at 11, the veteran American found the water at the par three 12th and the resulting double bogey dropped him back to seven under.
Nick Watney held the clubhouse lead on eight under following a dazzling 65, the best round of the week.
England's Ian Poulter, who slipped down the field on Saturday, picked up a shot on the third to move to seven under, before dropping shots at eight and ten to drift out of contention.
Earlier in the day, Spain's Miguel Angel Jimenez carded a stunning 66 to move into a tie for 12th on three under.
Ernie Els was another to make good progress, the South African closing with a 68 to end on one under.
Sixteen year old Matteo Manassero, who was already certain to claim the Silver Cup for leading amateur, carded a 72 to finish on four over, but there was disappointment for Spain's Sergio Garcia, who ended the week on ten over following a closing 78.
The leading groups may have had a number of holes still to play, but the crowd on the 16th had already witnessed two hole in ones from Nathan Green and Ryan Moore.