The MV-22B Osprey came down on its way to the remote Tiwi islands north of Darwin. Five of the marines are said to be in a serious condition.
They were taking part in Predators Run, involving 2,500 troops from the US, Australia, the Philippines, East Timor and Indonesia.
Only US personnel were on board the aircraft.
The incident took place on Melville Island north of the Northern Territory capital Darwin on Sunday.
Australian Prime Minister Anthony Albanese described the incident as tragic and said every assistance would be given to the injured.