Castelnovetto is a comune (municipality) in the Province of Pavia in the Italian region Lombardy , located about 50 km southwest of Milan and about 45 km west of Pavia. As of 31 December 2004, it had a population of 640 and an area of 18.5 km².

Castelnovetto borders the following municipalities: Ceretto Lomellina , Cozzo , Nicorvo, Robbio, Rosasco, Sant'Angelo Lomellina.