Rud Ab () is a city in, and the capital of, Rud Ab District of Sabzevar County, Razavi Khorasan province, Iran. It also serves as the administrative center for Frughan Rural District. The village of Qaleh Now-e Rud Ab was converted to the city of Rud Ab in 1996.
At the time of the 2006 National Census, the city's population was 3,470 in 883 households. The following census in 2011 counted 4,042 people in 1,229 households. The 2016 census measured the population of the city as 4,028 people in 1,254 households.