Rud Ab District () is in Sabzevar County, Razavi Khorasan province, Iran. Its capital is the city of Rud Ab.
At the time of the 2006 National Census, the district's population was 14,611 in 4,163 households. The following census in 2011 counted 13,515 people in 4,514 households. The 2016 census measured the population of the district as 13,406 inhabitants in 4,654 households.