Calatayud 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. Its population has been declining during the last decade due to migration.

Augusta Bilbilis was a city founded by the Romans in the province of Hispania Tarraconensis. It was the birthplace of famous poet Martial c. 40 AD. The modern town of Calatayud was founded near this Roman site.