Niau is a small atoll in French Polynesia, in the commune of Fakarava (Tuamotu archipelago). This atoll has a broad fringing reef, a diameter of 8 km and an area of 53 km2.

Niau's lagoon is swampy, hypersaline and entirely enclosed. The narrow strip of land surrounding the lagoon is covered by marsh vegetation. The enclosed lagoon area is 33 km2, making the land size at 20 km2. The lagoon has an unusual green color.