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.
"On the last few holes, I was not as calm as I was at the beginning of the round," Kaymer said. "But then in the playoff, I felt really relaxed and pretty laid back. That was probably the key that I could make a good stroke on the last putt."
Kaymer sank a putt from 18 feet on No. 18 at Le Golf National's Albatross Course to edge Ryder Cup star Westwood, who sent his approach shot into the water.
"When I hit it, I thought 'Oh it's going to be long,'" Kaymer said of his putt. "I kind of pulled it a little bit so that it sneaked in from the left side. It was good."
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.
"Sixty-five on this golf course under pressure is no easy round of golf," Westwood said.
Poulter came within two strokes of the lead but his momentum stalled 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 six 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 Westwood's 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 couldn't 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 under-13.
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 seven feet. He then had to hold his nerves and convert a three-foot par putt to force the playoff.
"I was a little up and down today," Kaymer said. "I had a good chance to close it earlier. I had especially a very good chance on 15, 16 and 17 and I couldn't make it. But I finally made a great shot on 18."