It is located in southwestern part of the country in the Varsinais-Suomiregion. The municipality had a population of 938 (2004-12-31) and covered an area of 61.52 km2 (excluding sea) of which 0.26 km2 is inland water. The population density was 15.31 inhabitants per km2.
Askainen was first mentioned in 1374 as Askais, when it was a part of the Nousiainen parish. It was included into the Lemu parish after its separation from Nousiainen sometime before 1404. Askainen was also held by the monastery of Naantali before the Reformation. Askainen was mentioned as a chapel community within Lemu in 1592, though chapel rights may have been granted earlier.
Askainen became a separate parish and municipality in 1910, but the parishes of Askainen and Lemu were reunited in 1960. Despite this, the two still acted as separate municipalities until 2009, when they became parts of Masku.