The attack happened at 11:42 am on the Highway No 4060 in Ban Dungssae village in Tambon Sakor of Narathiwat.
The slain ranger was identified as Maneethorn Kanchanakarn, 31.
Police said Maneethorn and 11 other rangers were patrolling the road on foot when insurgents detonated a bomb assembled with a gas cylinder to ambush them.
The two sides exchanged gunfire for about 10 minutes after the blast.
Maneethorn died on his way to a nearby hospital.