Casteggio is a comune, or municipality, located in the Province of Pavia within the Lombardy region of northern Italy. It is situated approximately 61 km (38 mi) south of Milan and about 25 km (16 mi) south of Pavia.
As of July 31, 2010, the municipality covered an area of 17.8 km² (6.9 sq mi) with a recorded population of 6,537 residents.