The roadside bomb struck an army vehicle carrying around 12 soldiers as it drove through the province of Yala, the deputy governor said.
The attack is one of the worst in the region in recent years.
Another five people were killed in a separate incident in Songkhla province, when gunmen fired at local Muslims at a mosque, reports say.
The soldiers caught in the blast in Yala were returning from a mission to negotiate with Muslim protesters in another district, a Thai Army spokesman said.
"After the troops moved them and were returning back to their base, a roadside bomb exploded, severely damaging the pickup truck and killing eight soldiers instantly," Colonel Acra Tiproch said.
Two other were reported to have died on the way to hospital.
In the nearby Sabayoi district of Songkhla, gunmen opened fire on a group of villagers after evening prayers at a mosque, a Thai army spokesperson said.
"The victims were all men in their 20s. They were killed inside the mosque," Colonel Tiproch said.
At least 2,000 people have died in Thailand's restive south since a resurgence of unrest in January 2004.
