Police said the victim was identified as Muhama Sobree Sarae, 40. He rode his motorcycle from his post to meet his wife in Sai Buri district.
Two gunmen on a motorcycle followed him, shooting him in the head with a handgun. Seriously wounded, he was sent to a nearby hospital but succumbed to his wound and was pronounced dead at hospital.
In nearby province of Narathiwat, a couple were wounded in gun attack on the Sungai Kolok-Tak Bai road and were sent to Sungai Kolok Hospital.
The victims were Sunan Pumee, 24 and his four months-pregnant wife Sira Yuso, 30.
Mr Sunan was shot in the arm and back while his wife was shot was wounded on her body. The baby was not hurt.
Police said the courple were on a motorcycle to Tak Bai district, and were followed by two attackers also on a motorcycle.
After the shooting, the gunmen fled the scene.