The minibus was travelling from Switzerland to Portugal when the accident happened just before midnight.
"The twelve passengers of the minibus, who were all Portuguese, are dead," a statement from the local authority said.
The minibus had swerved into the path of oncoming traffic and collided head-on with the truck.
The driver of the minibus and the two Italian drivers of the truck survived the crash but were injured, the local authority said.
A local village hall was being used as a temporary morgue.