Nias is an island located off the western coast of Sumatra, Indonesia. Nias is also the name of the archipelago of which the island is the centre, but also includes the Batu Islands to the southeast and the small Hinako Islands to the west. Nias Island covers an area of 5,573.27 km2 (2,151.85 sq mi). It is mostly a lowland area rising to around 800 m (2,600 ft) above sea level. There were 756,338 inhabitants on the island at the 2010 Census; at the 2015 Intermediate Census this had risen to 798,506 and the 2020 Census resulted in a total of 880,550. The official estimate as of mid-2023 was 930,294.

Sumatra is one of the Sunda Islands of western Indonesia. It is the largest island that is fully within Indonesian territory, as well as the sixth-largest island in the world at 475,807.63 km2 (182,812 mi.2), including adjacent islands such as the Simeulue, Nias, Mentawai, Enggano, Riau Islands, Bangka Belitung and Krakatoa archipelago.