The 40-year-old somehow managed to get past the security gate and board the Brussels-bound low-cost flight two days ago, Il Messaggero reported, but without confirming the incident.
He was caught only because he was sitting in another passenger's allocated seat.
As he was led off the plane by security staff, he reportedly said: “I like travelling and going home from time to time, but I didn't have money for the ticket”.
A spokesperson for Aeroporti di Roma, which operates the airport, was not immediately available to confirm the report, which did not identify the man by his real name or the airline, when contacted by The Local.
The alleged security breach comes at a time of heightened anxiety in Italy over a potential terrorist attack.
Fiumicino – Italy's biggest airport – handled almost 39 million passengers in 2014, more than twice that of Milan's Malpensa.
The airport descended into chaos in early May following a fire at terminal three, which handles international flights, causing flight cancellations and disruption for most of the summer.
But this is not the first time security has been breached at the airport. Weeks after the fire, three Algerians managed to force open the emergency exit of an Alitalia jet as it was heading for take-off.