Siamanna is a comune (municipality) in the Province of Oristano in the Italian region Sardinia, located about 80 kilometres (50 mi) northwest of Cagliari and about 15 kilometres (9 mi) east of Oristano.

Siamanna borders the following municipalities: Allai, Oristano, Ruinas, Siapiccia, Simaxis, Villaurbana.