Two bombs also exploded but caused no injuries Wednesday, less than a day after a pro-government Muslim official and his driver were slain when alleged insurgents raked their car with automatic weapons fire.
The head of Kamol Chuneth, a 65-year-old retired Buddhist school teacher, was found on a road in Pattani province, while his body was recovered from a nearby hut close to a rice field, said police Maj. Gen. Uthai Chaimala.
A note found with the head said: "You arrested an innocent man. I will get two lives in return."
The note apparently referred to the arrest last week of a Muslim student leader, Mahazee Boonthon.
Mahazee, a student at Pattani's Prince of Songkhla University, was arrested Friday on charges of murdering a provincial judge and involvement in continuing attacks in the southernmost provinces, which have claimed more than 860 lives since January 2004.
Mahazee, 28, has been detained in Bangkok pending formal trial, police said.
Also Wednesday, two bombs went off in Narathiwat, apparently aimed at security forces. No injuries were reported, local media reported.
The bombings came after Mahiza Zahi, a 43-year-old Pattani sub-district chief and government supporter, was killed along with his driver late Tuesday when gunmen sprayed their car with bullets, said police Col. Somsak Wannawak.
Kamol's beheading was thought to have been the fifth such incident in the area's latest wave of violence. The first beheading was in May 2004 in Narathiwat province.
Most attacks in the area have been drive-by shootings or bombings.
Pattani, Narathiwat and Yala provinces, bordering Malaysia, are among the only predominantly Muslim areas in Buddhist-majority Thailand. The government has generally blamed the attacks on the revival of a once-dormant separatist movement.
Southern Thai Muslims have complained for years of discrimination by the central government, particularly in jobs and education. Muslim elders have criticized the government's heavy-handed approach in trying to quell unrest in the south.
Prime Minister Thaksin Shinawatra has pledged to try peaceful means to resolve the long-standing conflict, but much of the area remains under martial law, further inflaming local resentment.