Castello dell'Acqua is a comune (municipality) in the Province of Sondrio in the Italian region Lombardy, located about 100 kilometres (62 mi) northeast of Milan and about 12 kilometres (7 mi) east of Sondrio. As of 31 December 2004, it had a population of 693 and an area of 13.9 square kilometres (5.4 sq mi).

Castello dell'Acqua borders the following municipalities: Chiuro, Ponte in Valtellina, Teglio.