COLOGNE, Germany, June 23, 2012 (AFP) - Overnight leader Willett had been three clear after eight holes of the final round, but saw his lead cut after bogeys at the ninth, 11th and 15th and finished on 73 to be 11 under for the tournament and tied with Fraser.
But he held his nerve on the fourth hole of the sudden-death play-off when he tapped in after Fraser had three-putted.
Joint-third, one behind were Ireland's Paul McGinley, Spaniard Gonzalo Fernandez-Castano and England's Chris Wood.
"It's amazing," said the former Walker Cup star, who had 19 top 10 finishes to his name going into this tournament.
"It was a tough day - it was brutal with the wind whipping and it was raining.
"Marcus had a great round today and I hung in there nicely. Hopefully I gave people a good show.
"This week has been strange, I've been working really hard with support from my family and my girlfriend. I've had some ups and downs in the last 18 months as everyone knows but I'm injury free now and back to playing well and I'd like to thank everyone back home for supporting me."
Fraser, looking for his third European Tour title, was left to regret straying down the right on the 18th in regulation play as he took three to reach the green.
"It was good fun," said the 33-year-old. "Danny is such a great guy and he's an awesome player - it's the first of many tournaments for him, that's for sure. It was nice to have a chance but unfortunately I couldn't do it.
"I've got to be pleased with the week. I played really well, three out of the four days and managed to make a few putts. It's just the way this game goes sometimes, you have to take it on the chin and get ready for the next week."
Leading and selected fourth round scores in the BMW International Open at Gut Larchenhof GC, Cologne, on Sunday (GBR/IRL unless stated, par 72):
277 - Danny Willett 65 70 69 73, Marcus Fraser (AUS) 64 74 68 71
- Willett won at fourth hole of play-off
278 - Chris Wood 65 70 70 73, Gonzalo Fdez-Castano (ESP) 71 69 69 69, Paul McGinley 65 70 77 66
279 - Marcel Siem (GER) 68 71 68 72, Henrik Stenson (SWE) 70 68 71 70
280 - Joel Sjoholm (SWE) 67 66 72 75, Ross Fisher 70 70 70 70, Thomas Bjorn (DEN) 69 70 70 71, Thongchai Jaidee (THA) 70 71 68 71
281 - Thomas Norret (DEN) 67 69 73 72, Andrew Marshall 71 67 70 73, Keith Horne (RSA) 66 73 67 75, Fabrizio Zanotti (PAR) 64 71 72 74, Simon Dyson 68 70 73 70, David Lynn 69 70 74 68
282 - Carlos Del Moral (ESP) 68 75 70 69, Steve Webster 69 70 69 74, Oliver Wilson 69 73 67 73, Pelle Edberg (SWE) 66 74 69 73, Mark Foster 69 74 70 69
283 - Mikael Lundberg (SWE) 68 74 68 73, Bernhard Langer (GER) 72 67 72 72, Shiv Kapur (IND) 66 70 75 72, Matthew Nixon 71 66 72 74, Paul Lawrie 72 70 72 69, Niclas Fasth (SWE) 68 69 77 69
Selected
284 - Sergio Garcia (ESP) 71 67 79 67
285 - Dylan Frittelli (RSA) 69 68 74 74
286 - Richard Green (AUS) 67 73 73 73, Richard Sterne (RSA) 69 69 69 79
287 - Colin Montgomerie 69 74 72 72
289 - Miguel Angel Jimenez (ESP) 68 74 75 72
290 - Thomas Aiken (RSA) 72 71 76 71, Rich Beem (USA) 71 70 76 73
291 - John Daly (USA) 68 73 77 73, Michael Campbell (NZL) 70 68 78 75
294 - James Kingston (RSA) 71 72 74 77