Allenby, who three weeks ago came from behind to deny Colin Montgomerie the English Open, this time grabbed a final-green birdie to catch Langer and then rammed in a 30-foot putt for a winning birdie when they went into sudden death.
The 38-year-old German, who only a fortnight ago was in the depths of despair because of a fourth attack of putting "yips", had charged two ahead with six birdies in his first seven holes and described his play over the opening 11 holes as "some of the best golf I've ever played''.
Even when Allenby forced a play-off, Langer was favourite to win it as he fired in his approach to 12 feet. But the man who has lifted at least one title on the European Tour every year since 1979 was suddenly faced with a putt to stay alive when Allenby holed his and it dribbled past.
The pair had tied on the 16- under-par total of 272, Langer scoring 66 and Allenby 69, and after picking up the pounds 100,000 first prize the 24-year-old from Melbourne, now third on this season's Order of Merit, said: "It was a lucky putt, but in play- offs things like that happen all the time.
"Bernhard played awesome and I'm very happy to see him playing well again. But I always felt I could win and it helped to know I've never lost a play-off." This made it three out of three.
"There was not a great deal I could do about Robert's finish, but I'm excited about the fact that I've not putted that well for a while,'' Langer said.
"After the US Open [where he missed the cut] I said that if this goes on for a long time I might consider retiring.''
A change of putters and a change of grip did the trick and Langer is now looking forward to this week's Irish Open and then the Open at Royal Lytham a fortnight later.
His start was sensational. A seven-iron to within a foot of the hole on the 415-yard first was followed by five successive birdie putts of between six and 12 feet from the third.
After eight holes he had seven threes and a birdie on the long ninth would have sent him to the turn in 29. Bunkered in two, however, he left a 12-foot attempt just short.
When he rifled a six-iron to five feet at the short 11th and made it he was looking unstoppable, but the 437-yard 12th cost him a double-bogey six and "probably cost me the tournament".
Seve Ballesteros had the worst closing round, a 78 which sent him tumbling to one under and 57th place.
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