Ski is a town in Nordre Follo municipality in Viken. The settlement had a total of 20,144 inhabitants as of 1 January 2020 and extends into Ås municipality. Ski is the administration center for Nordre Follo municipality. In 2004, the municipal council in Ski chose to give the town city status.

The eastern and western lines of the Østfold line meet at Ski, and the place therefore became from 1878 a communication center for parts of eastern Viken. Ski is said to be a typical station town. A typical feature of the town is its extensive use of brick, which is repeated in the largest apartment buildings as well as Ski Storsenter and the town hall. In the village are among other things Ski station, a hospital and Ski stadium. There is an industrial area on the outskirts of the town. Ski Storsenter was expanded in 2003 with a large number of shops, a large cinema with eight halls, an expanded library and Thon Hotell, and is the first so-called multi-center in Norway. On 15 October 2008, the center was again expanded with access to a new floor.

The name "Ski" comes from the old form Skeidi which is derived from the word skeid. Skeid means a place where you hold races . The three horse heads in the municipal coat of arms of the old Ski municipality symbolize this origin.