Rjukan is a city and administration center in Tinn municipality in Vestfold and Telemark county. The town is located in Vestfjorddalen, halfway between Møsvatn and Tinnsjå, and is named after Rjukanfossen ("the smoking waterfall") west of the town. The place was originally called Saaheim, but in 1912 the new town was named Rjukan by Tinn county council. Tinn municipal council approved the city status for Rjukan in 1996. The city has 3,101 inhabitants as of 1 January 2020. The nearest cities are Notodden (75 km) and Kongsberg (88 km). Byelva Måna flows through the city, all the way down to Tinnsjøen which is part of the Rjukan watercourse. Work is now being done on water levels up the entire river. Rjukan is located in Eastern Norway and is the region's 33rd largest city out of 37 cities.

Rjukan is on the UNESCO World Heritage List. Enrolled at the UNESCO World Heritage Committee's 39th session in Bonn as Rjukan - Notodden Industrial Heritage 5 July 2015.

Norsk Hydro began power and industrial development on the site in 1907. In 1910, Rjukan had more than 2,000 inhabitants, in 1920 more than 8,000 and in 1960 more than 6,500. Rjukan is considered one of the first cities in Norway to be planned from the ground up and designed by architects. Even the mailboxes and park benches were specially designed.