"The suspects opened fire on the policemen while they were in a pickup truck," Internal Security Operations Command spokesman Colonel Pramote Prom-in said.
"When the truck stopped, the suspects pulled them out and finished them off, taking their guns and mobile phones," he said.
The incident occurred at 10.40amin Tung Yang Daeng district of Pattani, 770 kilometres south of Bangkok.
The five policemen were on a mission to collect information on oil smuggling in the South, one of the main illicit activities in the restive region, Pramote said.
Pattani is one of Thailand's three southern provinces that have suffered violence since 2004 that has resulted in at least 5,300 deaths.
The insurgents are seeking autonomy in the majority-Muslim region in predominantly Buddhist Thailand.