The blast, which also wounded four other soldiers, struck an army truck in Pattani province shortly before midday, the authorities today said.
"A 21-year-old sergeant and a 27-year-old lance corporal were killed,' Police Major Somjet Thongpan told AFP by telephone.
"One of them died on the spot and the other passed away in hospital," he added.
Over the past week more than a dozen security personnel have been killed in the restive southern region, where a shadowy conflict waged by Muslim militants has left more than 5,700 people dead since 2004.
More From This Section
Earlier in the week five police officers were killed in an ambush and two soldiers perished in an attack at a school.
The violence comes despite several rounds of peace talks with the rebels, who are seeking some form of autonomy for the region, which was annexed by Buddhist-dominated Thailand a century ago.
Thailand's chief negotiator said last week the negotiations should continue despite the recent wave of bloodshed.
Talks began on March 28 in neighbouring Malaysia but have so far failed to halt near-daily violence, raising questions about the rebel leaders' influence over grassroots insurgents.