Police stopped the truck in heavy traffic on the A1 highway near the city of Frosinone after receiving calls from alarmed motorists who had spotted the man beneath the truck and feared he would be killed.
The man, believed to be around 20, was taken to hospital in Frosinone and was said to be suffering from exhaustion.
Police believe he strapped himself beneath the truck at the southern port of Brindisi while its Bulgarian driver went for a coffee break.
"The driver went to have coffee with other Bulgarians and in all probability was unaware the youth had meanwhile managed to stow away beneath his parked vehicle," police said.
The Afghan had not been detected by scanners at the port of Brindisi when the truck came off a ferry there, police said.
