Calatayúd, a town of central Spain, in the province of Saragossa, at the confluence of the rivers Jalón and Jiloca, and on the Madrid-Saragossa and Calatayúd-Sagunto railways. Pop. (1900) 11,526. Calatayúd consists of a lower town, built on the left bank of the Jalón, and an upper or Moorish town, which contains many dwellings hollowed out of the rock above and inhabited by the poorer classes. Among a number of ecclesiastical buildings, two collegiate churches are especially noteworthy. Santa Maria, originally a mosque, has a lofty octagonal tower and a fine Renaissance doorway, added in 1528; while Santo Sepulcro, built in 1141, and restored in 1613, was long the principal church of the Spanish Knights Templar. In commercial importance Calatayúd ranks second only to Saragossa among the Aragonese towns, for it is the central market of the exceptionally fertile expanse watered by the Jalón and Jiloca. About 2 m. E. are the ruins of the ancient Bilbilis, where the poet Martial was born c. A.D. 40. It was celebrated for its breed of horses, its armourers, its gold and its iron; but Martial also mentions its unhealthy climate, due to the icy winds which sweep down from the heights of Moncayo (7705 ft.) on the north.

In the middle ages the ruins were almost destroyed to provide stone for the building of Calatayúd, which was founded by a Moorish amir named Ayub and named Kalat Ayub, "Castle of Ayub." Calatayúd was captured by Alphonso I. of Aragon in 1119.