The dead included Major General Nopporn Ruanchan, who was commander of the army's 4th Infantry Division, said Lieutenant General Somsak Ninbanjerdkun.
Poor visibility due to the weather may have caused the crash, said Somsak, who is commander of the Third Area Army overseeing Thailand's north.
The UH-72 light helicopter went down in Chiang Mai, near Thailand's highest mountain, he said. It was returning to base in Phitsanuloke from a flood mission near the border with Myanmar.
The others on board were two pilots and two mechanics, Somsak said.