Dedicated to St Michael, it was built in the Middle Ages, between the end of the 4th and the beginning of the 5th century, on an earlier Christian site. As it stands today, the church is the result of several alterations. The lower central part of the façade dates back to the 11th century, while the two side portals date back to the 13th century and the central one to 1669. The bell tower, reconstructed between 1391 and 1395 on the basis of an older Romanesque tower, is an important example of late Gothic construction. The three-nave interior features a monumental 19th-century organ with a 17th-century case, a votive shrine from 1456, and a 16th-century fresco depicting a crucifix with saints in the central apse. The ceiling of the nave is decorated with frescoes from the 19th century. Restoration work carried out in the 1960s revealed remnants of the early mediaeval apse and Carolingian crypt beneath the presbytery. The well-preserved 5th-century baptistery outside the church is considered the main early Christian monument in Liguria. Access is from the loggia of the Municipal Palace with a staircase that descends to the level of the 5th century city, much lower than now, just one metre above sea level. Of particular appeal are the barriers at the windows, made of perforated sandstone with motifs characteristic of Lombard art, the early mediaeval tombs also with Lombard decorations, the basin where people were submerged to be baptised, and the niche that holds a fifth- or sixth-century mosaic depicting the Trinity enveloped by the Apostles in the form of doves. It is the only remaining mosaic of Byzantine character in northern Italy, with the exception of Ravenna.

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