GERMANY, 27 Sept 2015, (AFP) - The 45-year-old former paratrooper carded a final round, bogey-free 67 for a tournament total 17-under par 267.
England's Graeme Storm also enjoyed a final day 67 for a four-round total 268.
Storm's challenge unravelled when he found water from the 17th tee and, although he managed to save par at the last, Thongchai claimed victory at the 18th, becoming the oldest champion in the event's history.
"It's a very special week for me," said the Thai star.
"It is the third time I have won outside of Asia, so I'm very excited about that and I'm happy to win this tournament.
"I'm feeling very confident at the moment. I didn't make many mistakes this week -– only one bogey all week. I played very nicely and 17 under is some score for me.
"I liked the golf course. It's not very long and you have to keep it in play."
Despite missing out on a first European Tour since 2007, Storm was happy with his efforts.
"To finish second like last year is disappointing but I've secured my Tour card and I've got a job again for next year," Storm said.
"That was the main thing at the start of the week and today as well. Maybe I could have holed a few more putts and the title would be mine, but congratulations to Thongchai, he made a great up and down on the last."
Leading European Open final round scores at the Golf Resort Bad Griesbach on Sunday (GBR/IRL unless stated, par 71):
267 - Thongchai Jaidee (THA) 68 68 64 67
268 - Graeme Storm 65 67 69 67
270 - Pelle Edberg (SWE) 67 71 63 69
271 - Scott Hend (AUS) 69 69 68 65
272 - Lucas Bjerregaard (DEN) 68 68 68 68, Ross Fisher 67 65 69 71, Jamie Donaldson 67 68 67 70, Rikard Karlberg (Swe) 68 71 66 67
273 - Soren Kjeldsen (DEN) 66 70 67 70, Richard Bland 65 70 71 67, John Parry 66 70 70 67
274 - Peter Uihlein (USA) 68 68 67 71, Magnus A Carlsson (SWE) 67 68 67 72, Richard Green (AUS) 65 72 70 67, Mikko Korhonen (FIN) 70 67 68 69, Benjamin Hebert (FRA) 64 72 68 70, Darren Fichardt (RSA) 65 69 70 70, James Morrison 67 69 70 68
275 - Peter Hanson (SWE) 68 70 66 71, Charl Schwartzel (RSA) 66 66 75 68, An Byeong-Hun (KOR) 68 65 73 69, Simon Wakefield 69 70 68 68, Seve Benson 68 70 69 68
276 - Bernhard Langer (GER) 66 71 70 69, Edouard Espana (FRA) 69 69 70 68, Hennie Otto (RSA) 67 70 68 71, Alejandro Canizares (ESP) 70 67 70 69, Gregory Havret (FRA) 69 70 67 70, Raphael Jacquelin (FRA) 70 67 69 70