Both golfers finished 72 holes with totals of 13-under-par 271.
In the fourth round, Westwood vaulted from a tie for seventh and shot 6-under 65.
Kaymer shot 3-under 68 and missed several late chances that could have given him the win in regulation. He failed to sink birdie putts at the 16th and 17th holes. Kaymer atoned by sinking a 30-foot par putt on the 18th green to win the playoff.
"This is very special for me to get this trophy today," the 24-year-old Kaymer said. "I had some good chances, but finally I won it, so it's good."
Ian Poulter (67) was alone in third at 10-under 274.
Second- and third-round leader Rafa Echenique shot 6-over 77 Sunday, and at 5-under 279, finished in a tie for 13th place, eight shots back.