Dibaj (Persian: ديباج) is a city in the Central District of Damghan County, Semnan province, Iran. It was the administrative center for Rudbar Rural District before being transferred to the city of Kalateh Rudbar.
Dibaj is 55 kilometres (34 mi) north of Damghan, in the heart of the Alborz mountains. It is situated along the Mazandaran road, and is the main town in northern Damghan County. It was formerly named Chahardeh, a set of villages which was elevated to township status in 1995.