Masainas is a comune (municipality) in the Province of South Sardinia in the Italian region Sardinia, located about 45 kilometres (28 miles) southwest of Cagliari and about 15 km (9 mi) southeast of Carbonia. It is part of the Sulcis traditional region.

Masainas borders the following municipalities: Giba, Piscinas, Sant'Anna Arresi, Teulada.