Police said insurgents detonated a bomb planted next to a supporting pole of a railway bridge in Burangae village in Tambon Bukit in Joh I Rong district at 7.30 am.
Somchai Khamjun, a ranger with the 48th Ranger Regiment, was killed. The two injured rangers were identified as Matohe Sae and Itthipol Thongsakul.
A Muslim man, who was drinking tea in a nearby teashop, was also injured. He was identified as Ahama Jehmae.
The blast did not damage the pole but caused a hole 20cm deep and 30cm wide. Police said the bomb was detonated with a walkie-talkie.
A Muslim man told police that he spotted an suspicious object left at the supporting pole when he was crossing the railway to the teashop so he alerted a patrol unit of 20 rangers, who were passing by.
The bomb exploded as the rangers checked the object. Police suspect the insurgents were staked out nearby and detonated the bomb with the intention of killing the rangers.