The young man, estimated by the authorities to be around 20 years old, was arrested after other motorists on the Naples-Rome stretch of the A1 motorway alerted police after seeing him suspended from his makeshift harness.
It was unclear when exactly he had attached himself to the underside of the vehicle but police believe he had travelled like that from the southeastern port of Brindisi, 400 kilometres from where the lorry was intercepted.
The Bulgarian lorry drivers, who were on a trip from Turkey to Spain, said they had been unaware of the stowaway and were allowed to continue their journey.
The exhausted Afghan was briefly hospitalised before being released to a migrant reception centre. He was ordered to leave Italy within seven days after indicating he did not want to register an asylum request in the country.
More than 100,000 migrants have arrived in Italy since the start of the year. Many of them try to avoid registering there in the hope of being able to deposit asylum requests in northern Europe.