Arapaoa Island is the second-largest island in the Marlborough Sounds, at the north-east tip of the South Island of New Zealand. The island has a land area of 75 km2. Queen Charlotte Sound defines its western side, while to the south lies Tory Channel, which is on the sea route between Wellington in the North Island to Picton. Cook Strait's narrowest point is between Arapaoa Island's Perano Head and Cape Terawhiti in the North Island.

The Marlborough Sounds are an extensive network of sea-drowned valleys at the northern end of the South Island of New Zealand. The Marlborough Sounds were created by a combination of land subsidence and rising sea levels. According to Māori mythology, the sounds are the prows of the many sunken waka of Aoraki.