Sergeants Akkaraphol Phuwadolworanat and Warongkorn Sri-ngam were removing mines left during previous conflicts in the border district of Kantharalak, Si Sa Ket. Kantharalak was the site of a decades-long dispute over territory around the ancient Angor temple of Preah Vihear.
Akkaraphol died and Warongkorn was seriously injured when a mine exploded at 10.30am on Thursday.
Prime Minister General Prayut Chan-o-cha expressed condolences to the families of both soldiers and instructed commanding officers to use extra caution for the rest of the mission.
Authorities in Thailand and Cambodia met on December 8 and agreed to accelerate mine clearance efforts along the border to ensure safety for local villagers.
The Army will hold funeral rites for Akkaraphol and provide benefits to the families of both soldiers as per regulations.
The Defence Ministry says it has so far cleared landmines from 2,527 square kilometres in 21 provinces. About 29.7 square kilometres in six provinces still needed to be swept.