The patron saint of Diano Castello is Saint Nicholas, celebrated on 6 December. The parish church of San Nicola di Bari, in the old town centre, was built in Baroque style by Giovan Battista Marvaldi between 1699 and 1725; it holds a marble inlaid altar, a wooden crucifix by Anton Maria Maragliano and the 18th-century choir.
The town was built around the 10th century under the name Castrum Diani, as a defensive site against Saracen pirate raids; mentioned in a document from 1033 as a dependency of the County of Albenga, in the early 11th century it became a domain of the Clavesana family. As pirate raids gradually declined and coastal communities grew, above all Diano Marina, the town's importance diminished along with its population; the 1887 earthquake killed 32 inhabitants and caused serious structural damage to houses and monuments. Near the main square the ruins of the castle of the Clavesana marquises still stand.
