Castel del Rio is a comune (municipality) in the Province of Bologna in the Italian region Emilia-Romagna, located about 35 kilometres (22 mi) southwest of Bologna.

Historically, the town's countryside is a large producer of chestnut, which has received the European Protected Geographical Status.

Traces of human presence in the area date to the 6th-5th centuries BC. The current town was however founded in the 5th-6th centuries AD as Massa di S. Ambrogio. Starting from the 10th century, there were fortifications and castles, whence the toponym Castrum Rivi from which the current name derives. In 1076 the castle was acquired by Matilde of Canossa; later Emperor Otto IV gave the fief to the Alidosi family, who held it for more than four centuries until it became part of the Papal States.

During World War II Castel del Rio was located across the Gothic Line. Numerous of its citizens fought as partisans against the German occupation.

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