Sayansk is a Russian town (since 1985) in the Irkutsk Region of Russia.

It is located on the Irkutsko- Cheremkhovskaya Plain, on the Oka River (the basin of Angara), 270 km to the northwest of Irkutsk. It stands at the 25 km distance from the Zima railway station.

The town with the population of 39 453 people (as of 2013) takes the overall area of 83 sq km.

Residential districts of the town are distanced 12 km away from its industrial centre.

It was founded as the settlement of Sayansk in 1970 and became an industrial township in 1975. Sayansk has been a town since 1985. It was named after its location by the East Sayan spurs.