Tanahmasa Island is one of the Batu Islands, an archipelago of Indonesia located in the Indian Ocean off the west coast of Sumatra, between Nias and Siberut. The three primary islands, of approximately equal size, are Pini, Tanahmasa, and Tanahbala. There are forty-eight smaller islands, of which less than half are inhabited. The islands are governed as a part of North Sumatra province. In Indonesian and Malay, batu means rock or stone.
The people of the Batu Islands have had substantial interaction with the populations of Nias, to the north. The islands have occasionally been a destination for slaves who escaped from Nias, and in the past decade have become a destination for surfing boat charters from Padang, on the Sumatran mainland.
The equator passes through the archipelago, north of Tanahmasa and south of Pini.