Costarainera (Ligurian: A Costarainea) is a comune (municipality) in the Province of Imperia in the Italian region Liguria, located about 100 kilometres (62 mi) southwest of Genoa and about 9 kilometres (6 mi) southwest of Imperia. As of 31 December 2004, it had a population of 775 and an area of 2.4 square kilometres (0.93 mi2).
Costarainera borders the following municipalities: Cipressa and San Lorenzo al Mare.