Masir (Persian: مصير) is a village in, and the capital of, Poshtkuh-e Mugui Rural District of Mugui District, Fereydunshahr County, Isfahan province, Iran.

At the 2006 National Census, its population was 423 in 74 households, when it was in the Central District. The following census in 2011 counted 332 people in 77 households. The latest census in 2016 showed a population of 298 people in 75 households.

After the 2016 census, Pishkuh-e Mugui and Poshtkuh-e Mugui Rural Districts were separated from the Central District in the establishment of Mugui District.