Sos del Rey Católico is a historic town located in the comarca of Cinco Villas, province of Zaragoza, Aragon, Spain.
The town was founded by Sancho I of Pamplona either in the year 907 or in 908, as a border town during the Reconquista.
It was the birthplace of King Ferdinand II (1452), in whose honor the town, formerly known simply as Sos, took the name del Rey Católico(of the Catholic King). It is one of the best preserved medieval cities in Aragon; large parts of the old town are said to look much like they did at the time of Ferdinand's birth. The many monuments in the town include: the Palacio de los Sada, birthplace of the Catholic Monarch; several churches, including the gothic church of Saint Stephen; the old medieval wall and seven of its gates; and the medieval stock exchange (Lonja).