A coast guard spokeswoman told AFP: "We received information at 14:47 (05:47 GMT) that a US military Osprey aircraft crashed off the island of Yakushima."
"We were also informed that there was a crew of eight on board. We don't have additional information at this time."
Yakushima Island is located south of Kyushu in Japan's southernmost portion.
The incident comes after another Osprey military plane crashed in northern Australia in August, killing three U.S. Marines out of 3 people on board.
A Boeing MV-22B Osprey crashed on Melville Island, north of Darwin, during a military exercise for local troops.