Atu (Persian: اتو) is a village in, and the capital of, Kaseliyan Rural District of Zirab District, Savadkuh County, Mazandaran province, Iran.

At the 2006 National Census, its population was 565 in 133 households, when it was in the Central District. The following census in 2011 counted 351 people in 129 households. The latest census in 2016 showed a population of 510 people in 185 households, by which time the rural district had been separated from the district in the establishment of Zirab District.