Police said the explosion occurred at 10.55am at Taloh village in Tambon Sakor of Narathiwat’s Si Sakhon district.
The five rangers were building a house for villagers and the insurgents apparently remotely detonated a homemade bomb planted in the middle of the house, police said.
The ranger killed in the blast was identified as Udomsak Raotapong.
The four injured were Thanasask Khirikkaew, 35, Adul Chairit, 32, Puteh Alee, 39, and Thirawat Kongjui, 27.
The injured were rushed to a nearby hospital.