Farg Qaleh (Persian: فرگ قلعه) is a city in, and the capital of, Farah Dasht District of Kashmar County, Razavi Khorasan province, Iran. It also serves as the administrative center for Qaleh-ye Bala Rural District.

At the 2006 census, the population of Farg was 2,477 in 615 households, when it was a village in Bala Velayat Rural District of the Central District. The following census in 2011 counted 2,459 people in 727 households. The latest census in 2016 showed a population of 2,134 people in 663 households.

In 2018, Qaleh-ye Bala Rural District and Rezqabad Rural District were established in the newly formed Farah Dasht District. The villages of Farg and Qaleh-ye Bala were merged to form the new city of Farg Qaleh.