Thongchai Jaidee poses with his trophy after winning the Nordea Masters Sunday at PGA National, Malmo, Sweden. (AP photo)
Thongchai birdied the par-5 18th at PGA Sweden's Lake Course while Gallacher and Dubuisson could only make par.
Earlier, the Lop Buri native hit six birdies and an eagle three on the 11th to finish in 7-under 65 and an overall 16-under 272.
Dubuisson could have avoided the playoff with a birdie on the 18th, but three-putted from just off the green.
Gallacher appeared to be out of the race when he bogeyed at No. 17, but he converted from 20 feet at the next.
"I worked hard today," Thongchai said. "I started with three birdies in a row, had another one on six, then a good comeback on 11 (where he made an eagle three).
"We were nervous the last few holes - 16, 17 and 18 are tough holes - but 16 under was good enough for a playoff.
"The golf course is wide open, you have to hit good golf shots and the weather helped a little bit." Bangkok Post