Masson Island or Mission Island is an ice-covered island about long and rising to , lying northwest of Henderson Island within the Shackleton Ice Shelf. Masson Island is located in the western part of Mawson Sea at and has an elevation of . Masson Island was discovered in February 1912 by the Australian Antarctic Expedition under Sir Douglas Mawson, who named it for Professor Sir David Orme Masson of Melbourne, a member of the Australian Antarctic Expedition Advisory Committee.