Abdulnasea Katoorae had his throat slashed by six armed men who stormed a Pattani province school on Thursday night and stole weapons from him and other guards, said police on Friday.
Thai authorities have trained volunteer civil guards in weapons use and supplied them with guns to help counter a wave of violence by suspected separatists in three majority-Muslim southern provinces which has left nearly 200 people dead this year.
The attack on the guard came hours after a home-made bomb exploded outside the home of the chief of Sungai Kolok district in the province of Yala. No one was injured in the blast, said police.
The explosion was apparently aimed at intimidating authorities and not meant to take anyone's life.
Three other small bombs exploded in Yala on Wednesday without causing casualties. Authorities believe they are aimed at creating fear and communal tensions in the region.
In Bangkok, Prime Minister Thaksin Shinawatra called a meeting of his top security officials to try to hash out new methods to quell the unrest in the south.
Thailand's south has endured decades of separatist violence. After a period of relative peace, trouble flared at the start of this year with bombings and murders targeting officials, security forces and Buddhist monks.
The violence reached a peak on 28 April when 108 Muslims, suspected of being Islamist separatists, were killed when they launched raids on police posts and checkpoints.
Three Buddhist temples were bombed in mid-May and an elderly Buddhist man was decapitated last month.
On Wednesday thousands of teachers in the south held a rally demanding the government provide better security after one of their colleagues was shot dead.