Sergeant Yaya Saepaeing was shot three times on Wednesday by a man riding on the back seat of a motorcycle in Narathiwat province.
The attack came just hours before an army commander was preparing to meet a rebel leader for peace talks, officials said.
"We believe the gunman was connected to the insurgent groups that have been causing trouble in the area," said police Lieutenant Pramote Plubplung.
The southern provinces of Narathiwat, Yala and Pattani - Muslim majority areas in this Buddhist kingdom - have been in the grip of almost daily shootings by unidentified gunmen. More than 100 people have been killed.
The government has blamed the attacks on Islamist separatists, reviving a decades old campaign.
On 28 April, government forces shot and killed 107 suspected rebels who attacked security outposts in coordinated attacks in several places in the south.
Thai Muslims, who are linguistically and culturally closer to Malays in Malaysia, complain that the government does not recognise their Islamic Malay culture and tries to suppress their language - discriminating against them in jobs and education.
Wan Kardir Che Man, the leader of Bersatu, an umbrella organisation of three separatist group, reportedly has said from exile in neighbouring Malaysia he has given up the demand for a separate state and is willing to talk to the government.
Lt Gen Pisarn Wattanawongkiri, the army commander of the southern provinces, said on Wednesday he would meet Wan Kadir soon "for an unofficial discussion".
Thai Muslims in three provinces
complain of discrimination
"The meeting is not meant to recognise his organisation but merely to know each other, to exchange views," Pisarn told reporters.
Prime Minister Thaksin Shinawatra said on Tuesday he would not accept any demand for autonomy for the south, which Wan Kadir had indicated as a possible solution.
Thaksin told reporters on Wednesday the talks were aimed at expressing "our intention that we want peace and prosperity for the people" in the south.
"This is not a recognition of a [an Islamic] state," he said.