Japan's coast guard said it had no further details of the incident, including the status of those on board.
A spokesperson for US forces in the region said they were still gathering information about the incident.
The crash happened on Wednesday afternoon, with local residents reporting the aircraft had fire blowing from its left engine as it fell into the sea, Japanese broadcaster MBC reported.
In August, a US Osprey crashed off the coast of northern Australia while transporting troops during a routine military exercise, killing three US Marines.
Another crash-landed in the ocean off Japan's southern island of Okinawa in December 2016, prompting a temporary US military grounding of the aircraft.
The tilt-rotor plane, which can fly both like a helicopter and a fixed-wing aircraft, is operated by the US Marines, US Navy and Japan's defence forces.
The deployment of the Osprey in Japan has been controversial, with critics saying the hybrid aircraft is prone to accidents.
The US military and Japan say it is safe.
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