The victim was identified as Ranger Jiraphan Sinsiadnian, 28, of Ranger Company 4914.
The shooting occurred about 10.30pm on a rural road at Ban Kadee in tambon Cherngkhiri, said Pol Sub-Lt Piyawat Attana, duty officer at Sisakhon police station.
Sgt Kasitpong Buasakad told police that a team of 12 rangers were on foot patrolling along the rural road when a gunman riding pillion on a passing motorcycle sprayed bullets at them with an assault rifle. The rangers fired back and the attackers sped away on their vehicle.
Ranger Jiraphan took two bullets in his torso and died on the spot.
Police blamed separatist militants.