Kyodo News quoted police as saying the serviceman allegedly trespassed into an apartment in the village of Yomitan in Okinawa prefecture and hit the 13-year-old boy early Friday.
Last month, two U.S. sailors in Okinawa were accused of attacking and raping a Japanese woman who was returning home. The commander of the U.S. naval forces in Japan later apologized for the incident and since then U.S. military personnel in Japan have been under a curfew from 11 p.m. to 5 a.m.
In another incident involving the U.S. military in Japan, a U.S. sailor was found dead at a train station in Nagasaki prefecture late last month. It was not clear if the death was due to an accident or foul play.
Kyodo said Friday's incident may further stoke local resentment in Okinawa against the U.S. military presence. There is already much opposition stemming from a recent decision to deploy MV-2 Osprey aircraft at a U.S. base there because of safety questions about the plane.