The nine-year-old boy was left lying in the street after crossing residential Whitehawk Road in Brighton on Sunday.
He was taken to Royal Sussex County Hospital where he was treated for serious head injuries.
The rider of the moped, who had a pillion passenger, failed to stop at the scene following the collision, which happened at around 12.15pm.
Police believe the vehicle is stolen, describing it as silver and black or dark red. It is suspected to have some damage as a result of the impact.
A police spokesperson said: “The nine-year-old boy was taken to the Royal Sussex County Hospital in Brighton with serious head injuries. He has since been discharged.
“The rider of the moped, who had a pillion passenger and was travelling south, failed to stop at the scene.”
Police said they were “continuing to carry out covert and overt operations in Brighton and Hove, targeting known motorcycle thieves”, and had made a number of arrests in recent weeks.
:: Anyone with any information about the collision is asked to report it online or call 101, quoting serial 502 of 16/09. Alternatively, you can visit the Crimestoppers website or contact the independent charity anonymously on 0800 555 111.