The girl was left on the bus after the morning journey to school in Civitavecchia and found two hours later by another bus company worker when the vehicle was parked at the depot, a major cruise and ferry port near Rome.
The girl was discovered only after her parents, who had accompanied her to the bus at 8.30am, received a call from the school saying she hadn't arrived, Il Mattino reported.
After a couple of anxious hours, she was found on the bus at the depot. Neither the driver nor assistant realized she had been left on board.
The parents reported the incident to Civitavecchia police and the pair are facing charges of child neglect.