Castelsardo is a town and comune in Sardinia, located in the northwest of the island within the Province of Sassari.

Castelsardo or Tempio is the episcopal see of the Roman Catholic diocese of Ampurias (not to be confused with its Catalan namesake) in Sardinia, suffragan of the archbishopric of Sassari. The city is one of several possible locations for the ancient city of Tibula.The Right Rev. Antonio Maria Contini, born 6 November 1839, was appointed Bishop of Ogliastra on 26 September 1882, and transferred to this diocese on 16 January 1893. Ampurias was erected in 1113; Cività, now Tempio, in 304 by St. Simplicius. Cività was united to Ampurias by pope Julius II in 1506. Later the see was transferred to Terranuova.

Pope Gregory XVI suppressed the cathedral there by the Papal Bull "Quamvis aqua", 26 August 1839, and raised the Collegiate Church of St. Peter, in Tempio, to a cathedral, uniting Tempio and Ampurias, so that one bishop should govern both. The see was vacant from 1854 to 1871.

In the early 20th century Ampurias, or Castelsardo, had 11,200 Catholics, 8 parishes, 25 secular priests, 5 seminarists, 34 churches or chapels. Tempio had 26,200 Catholics, 17 parishes, 44 secular priests, 6 seminarists, 71 churches or chapels.