Alastaro is a former municipality in the Southwest Finland region of Finland. It was merged into the town of Loimaa on 1 January 2009, and since that date it has formed part of Loimaa rather than existing as an independent municipality.
It is located in the south-western part of the country, within the region of Southwest Finland (Varsinais-Suomi). Its most notable characteristic is its administrative history: like many small Finnish municipalities, it was consolidated into a larger neighbouring town, in this case Loimaa.