Sain (Persian: سيين) is a village in Arjestan Rural District of Sabalan District, Sareyn County, Ardabil province, Iran.

At the 2006 census, its population was 839 in 179 households, when it was in Sabalan Rural District of the former Sareyn District of Ardabil County. The following census in 2011 counted 979 people in 264 households, by which time the district had been separated from the county in the establishment of Sareyn County. The latest census in 2016 showed a population of 947 people in 276 households. It was the most populous village in its rural district.