The patron saint of Ranzo is Saint Donatus of Arezzo, celebrated on 7 August. The parish church dedicated to him, in the hamlet of Costa Parrocchia, was a rural vicariate of the bishop of Albenga until 1964: it is a Baroque-era building with a single nave and six side altars, besides the high altar, holding a painting of the Crucifixion of Jesus by Orazio De Ferrari.
In the hamlet of Borgo di Ranzo stands the church of San Pantaleo, built in the 15th century over an earlier 11th-century Romanesque church: it holds a polyptych painted in 1554 by the local painter Giorgio Guido da Ranzo, while the portico is decorated by the same Guido da Ranzo family and by portals of the Lapicidi di Cenova school from 1493. In the hamlet of Bacelega stands instead the parish church of Nostra Signora dell'Assunta, built in the 15th century with a 16th-century portal, which holds paintings by Pietro Guido, Giorgio's father: the place name Bacelega is thought to derive from the Greek Basilikos, indicating the presence of an ancient Benedictine abbey between the 6th and 7th centuries. The territory was long disputed between the marquises of Clavesana, the Del Carretto family and the Republic of Genoa, which finally gained control of it in 1393, placing it under the captaincy of Pieve di Teco until the Napoleonic domination.
