Gach Sar (Persian: گچسر, also Romanized as Gach-i-Sar and Getschesar)[1] is a village in Nesa Rural District, Asara District, Karaj County, Alborz Province, Iran.

[2] Located on the main road from Tehran to Chalus on the Caspian coast, 7 km south of the Kandavan Tunnel, Gach Sar has farmlands irrigated by the Karaj River, producing fruits, as well as plants for animal fodder.

[3] Gach Sar has a gypsum mine, and a hotel built during the reign of Reza Shah Pahlavi as a royal lodge.

[3] Naturally occurring local plants include marjoram, bugloss, and marshmallows.

Wild animals include jackals, wolves, foxes, boars, and bears.