Calatayud (; 2014 pop. 20,658, declining along the last decade due to migration) is a municipality in the Province of Zaragoza, within Aragón, Spain, lying on the river Jalón, in the midst of the Sistema Ibérico mountain range. It is the second-largest town in the province after the capital, Zaragoza, and the largest town in Aragón other than the three provincial capitals. It is the seat of the comarca of Calatayud.