Five plainclothes police officers were slain by gunmen in Pattani's Thung Yangdaeng district on Wednesday, police said.
A vehicle carrying five or six men followed the pickup truck carrying the policemen and attacked them on a road in the southern border province, Pol Col Kowit Rattanachote said.
He said the police pulled over and attempted to take cover when the killers started firing at their vehicle.
The bodies of four policemen were found next to the vehicle and another was found inside.
The officers belonged to a regional police squad that investigates oil smuggling.
Pol Col Kowit said the militants searched the police vehicle and stole their victims' weapons before fleeing in a pickup truck.
More than 5,000 people have been killed in predominantly Buddhist Thailand's three Muslim-dominated southernmost provinces of Narathiwat, Pattani and Yala since an Islamic insurgency erupted in 2004.