MALMÖ, June 1, 2014 (AFP) - Thongchai, lying four shots off the lead overnight, carded a closing 65 to take the clubhouse lead with a 16-under par total of 272.
He then had to wait for the last four groups to finish, with France's Dubuisson and Briton Gallacher matching his score.
On the first trip back down the 18th, Thongchai spun his third shot to three feet and, after seeing Dubuisson and Gallacher miss lengthy birdie putts, converted to seal a sixth European Tour title.
"I worked hard today," said Thongchai.
"I started with three birdies in a row, had another one on six, then a good comeback on 11 (where he conjured up an eagle).
"We were nervous the last few holes -- 16, 17 and 18 are tough holes -- but 16 under was good enough for a play-off."
Joint overnight leaders Eddie Pepperell and world number two Henrik Stenson's bid faded away -- Stenson bogeying the last and Pepperell finding water to also finish with a six.
Dubuisson would have won with a birdie on 18 in regulation play, but three-putted from just off the green.
Gallacher's chance looked to have gone when he bogeyed the 17th from a greenside bunker, but he converted from 20 feet at the next.
Dutchman Robert-Jan Derksen matched Thongchai's 65 to take fourth on 15 under, with Stenson alone in fifth and Pepperell sharing sixth with Spain's Alvaro Quiros and another local favourite in Robert Karlsson -- whose 63 was the best round of the week.
Leading fourth round scores in the EPGA Nordea Masters in Malmo on Sunday (GBR & IRL unless stated, par 72):
Thongchai Jaidee wins playoff on first extra hole.
272 - Thongchai Jaidee (THA) 69 70 68 65, Victor Dubuisson (FRA) 69 69 67 67, Stephen Gallacher 67 72 65 68
273 - Robert Jan Derksen (NED) 70 71 67 65
274 - Henrik Stenson (SWE) 69 70 64 71
275 - Robert Karlsson (SWE) 70 70 72 63, Eddie Pepperell 66 72 65 72, Alvaro Quiros (ESP) 71 67 66 71
276 - David Howell 73 70 67 66, Rafael Cabrera Bello (ESP) 68 72 67 69
277 - Romain Wattel (FRA) 71 70 66 70, Maximilian Kieffer (GER) 70 74 67 66, Julien Quesne (FRA) 72 71 70 64
278 - Thomas Pieters (BEL) 73 69 68 68
279 - Thomas Bjorn (DEN) 70 71 68 70, Rikard Karlberg (SWE) 67 75 68 69, Darren
Fichardt (RSA) 73 71 68 67
280 - Craig Lee 68 74 71 67, Michael Hoey 69 75 69 67, Edoardo Molinari (ITA) 71 69 73 67
281 - Marcus Kinhult (SWE) 71 72 69 69, Eduardo De La Riva (ESP) 72 72 70 67, Tom Lewis 71 71 68 71, Simon Dyson 69 72 70 70
282 - Peter Hanson (SWE) 69 71 73 69, Shane Lowry 71 73 72 66, Jorge Campillo (ESP) 68 76 70 68, Bradley Dredge 69 70 67 76, Christopher Doak 70 69 69 74, Raphael Jacquelin (FRA) 72 70 69 71, Alexander Levy (FRA) 72 73 71 66
283 - Soren Kjeldsen (DEN) 70 75 70 68, Felipe Aguilar (CHI) 70 75 70 68, Mikko Ilonen (FIN) 70 71 71 71, Niclas Fasth (SWE) 71 70 70 72, Jack Doherty 70 74 68 71, Matteo Manassero (ITA) 74 70 70 69, Nacho Elvira (ESP) 73 71 68 71, Pablo
Larrazabal (ESP) 70 72 68 73
284 - Paul Waring 68 76 70 70, Kevin Phelan (USA) 71 70 73 70, Andrea Pavan (ITA) 74 71 71 68, Richard Green (AUS) 72 69 76 67, Peter Lawrie 75 70 72 67, James Morrison 70 73 66 75, Oliver Fisher 69 75 69 71
285 - Jose-Filipe Lima (POR) 71 74 69 71, Lucas Bjerregaard (DEN) 70 75 70 70, Daniel Im (USA) 72 72 74 67, Wade Ormsby (AUS) 69 74 72 70, Francois Calmels (FRA) 73 70 75 67, Johan Carlsson (SWE) 68 77 71 69
286 - Richie Ramsay 72 72 71 71, James Kingston (RSA) 72 70 72 72, Scott Jamieson 69 72 73 72, Bjorn Akeson (SWE) 71 73 75 67
287 - Jens Dantorp (SWE) 66 73 71 77, Edouard Dubois (FRA) 70 73 68 76, Daan Huizing (NED) 67 78 70 72
288 - Mikael Lundberg (SWE) 69 75 72 72, Shiv Kapur (Ind) 76 68 72 72, Sebastian Soderberg (SWE) 67 76 74 71, Kristoffer Broberg (SWE) 70 74 68 76
289 - Jeev Milkha Singh (IND) 71 74 70 74, Matthew Baldwin 71 73 73 72, Gregory Havret (FRA) 71 74 70 74
290 - Emiliano Grillo (ARG) 73 72 71 74, Gareth Maybin 67 75 74 74, Fredrik Andersson Hed (SWE) 74 70 74 72
291 - Miguel Angel Jimenez (ESP) 69 73 75 74
292 - Ricardo Gonzalez (ARG 69 73 75 75
293 - Stuart Manley 69 74 77 73
298 - Adam Gee 68 75 76 79