Chalus County (Persian: شهرستان چالوس) is in Mazandaran province, Iran.

[9] During the Pahlavi dynasty, a group of Langrudi people migrated from western Gilan to Chalus.

The Eastern Gilaki language is spoken in the entire Chalus River valley, though some Kurdish tribes were established in Kojur and Kelardasht in the Qajar period.

[3] Chalus County's population history and administrative structure over three consecutive censuses are shown in the following table.

The warm, humid climate of Chalus was formerly considered unhealthy, and the bulk of the population of Chalus, as of all other towns in this region, used to migrate in summer to the summer pastures on the slopes around Delir and in the Kelardasht basin, one of the most popular and longest-settled districts in the region.