"A group of about 200 backyard chicken began falling ill and dying on November 3," Sakchai Sriboonsue, director general of the livestock department, told AFP.
"We were notified two days later and the official test result today confirmed that it's H5N1," he said, adding that all suspected poultry had now been slaughtered.
Thailand, the world's fourth-largest exporter of poultry, has slaughtered millions of birds to halt the spread of the bird flu virus, which was last found in poultry in January.
Since then the kingdom had been declared free of the virus.
Thailand's public health ministry said Thursday that no human-form of bird flu had been found here since the last confirmed case in August 2006.
At that time there were 25 confirmed human cases, 17 of whom died.