Castelnovetto is a comune, or municipality, located in the Province of Pavia within the Lombardy region of northern Italy. It is situated approximately 50 km (31 mi) southwest of Milan and about 45 km (28 mi) west of Pavia.
As of December 31, 2004, the municipality covered an area of 18.5 km2 (7.1 sq mi) and had a population of 640 residents.