Nagaoka (長岡市, Nagaoka-shi) is a city located in Niigata Prefecture, Japan. It is the second largest city in the prefecture, after the capital city of Niigata. As of 4 August 2021, the city had an estimated population of 264,611 in 109,283 households and a population density of 300 inhabitants per square kilometre (780/sq mi). The total area of the city was 891.06 square kilometres (344.04 sq mi).