Sado is a city located on Sado Island in Niigata Prefecture, Japan. Since 2004, the city has comprised the entire island, although not all of its total area is urbanized. Sado is the sixth largest island of Japan in area following the four main islands and Okinawa Island. As of June 1, 2023, the city has an estimated population of 48,195 and a population density of 56.3 inhabitants per square kilometre (146/sq mi). The total area is 855.69 square kilometres (330.38 sq mi).

Sado Island is an island located in the eastern part of the Sea of Japan, under the jurisdiction of Sado City, Niigata Prefecture, Japan, with a coastline of 262.7 kilometres (163.2 mi). In October 2017, Sado Island had a population of 55,212 people. Sado Island covers an area of 854.76 km2 (330.02 sq mi), and is the second largest island after Okinawa Island outside of the four main islands of Japan, excluding the disputed Southern Kurils. The shortest distance between Sado Island and Honshu is 32 km (20 mi). The highest peak on Sado Island is Mount Kinpoku, with an elevation of 1,172 m (3,845 ft).