The 2007 Rookie of the Year on the European Tour kept his composure in a dramatic finish to clinch his third title after having missed chances on the last few holes.
Kaymer sank a putt from 18 feet on No. 18 at Le Golf Nationals Albatross Course to edge Ryder Cup star Westwood, who sent his approach shot into the water.
Westwood had the round of the day with seven birdies and a bogey for a 6-under 65 that forced a playoff at 13-under 271.
Ian Poulter of England carded a 67 to take third place with a 274 total, one stroke better than Anders Hansen of Denmark, who tied for fourth with Swedens Peter Hanson.
Poulter came within two strokes of the lead but stalled his momentum with two bogeys in the last four holes.
Going into the final round in second place, Kaymer converted a 10-foot birdie putt on the sixth to tie overnight leader Rafa Echenique of Argentina at 12 under.
But on the next hole, both missed par putts, letting Westwood share the lead at 11 under as the Englishman hit a birdie from 6 feet on the ninth.
Westwood struggled with his putting last year but his birdie putts Sunday helped him climb from seventh place after the third round to tie for first.
Kaymer went one stroke up on Echenique on the eighth with an outstanding birdie putt from more than 50 feet before Westwood holed a 20-foot putt on the 11th to stay neck-and-neck with the German.
From then on, it became a nerve-racking contest between Westwood and Kaymer, who both added two more birdies to increase their lead to three strokes at 14 under while Echenique, after his bogey on the seventh, slipped further down the leaderboard with two triple-bogeys on the back nine.
Though Westwoods putting was steady, the Englishman was let down by a tee shot on the 14th that landed in the rough. He failed to get his approach shot on the green and made a bogey on the longest hole of the Albatross course.
But Kaymer couldnt capitalize, hitting his tee shot into the bunker at the 12th and dropping a shot to fall back into a tie for the lead with Westwood at 13 under.
The German nearly blew his chances on the 15th, where his tee shot stopped just three feet from the water.
Westwood wound up his last four holes with solid pars and waited inside the clubhouse for Kaymer to finish his round.
Kaymer had another opportunity to clinch the title on the 17th, but missed a birdie putt from 7 feet. He then had to hold his nerves and convert a 3-foot par putt to force the playoff.