Name
The original name Mustasaari "Black Island" may have been a medieval island cleared by a wildfire or an island that looks dark when approaching from the sea. Due to isostatic uplift, the area referred to is now inland. The parish is first mentioned as Mustasaari sockn (parish of Mustasaari, in Finnish) in 1348. In Swedish, the pronunciation developed into Mussor, although the form Mustasaari remained in use in the community. Mikael Agricola, the founder of written Finnish, spelled it Mustsåår in 1530. In 1606–1611, it was known as Mussar, but in 1611 the city of Vaasa was founded in the parish and thus the parish was known as Vaasa. In 1772, it was known as "city of Vaasa and Mustasaari annex" (Vaasan kaupunki ja Mustasaaren anneksi), and in 1807–1867 "parish of Vaasa and Mustasaari" (Vaasan ja Mustasaaren seurakunta). Since then, Vaasa has been an independent parish. The municipality was named Korsholm in Swedish in 1927, after the medieval Korsholma castle.
Middle Ages
Korsholm has a history that can be dated back to 1348. In that year Korsholm was mentioned for the first time in writing in a royal letter concerning freedom of commerce. Therefore the municipality celebrated its 650th anniversary in 1998. In the mid-14th century Saint Mary's Church was built in Korsholm island. The whole of Ostrobothnia was governed for hundreds of years from Korsholma Castle (Chrysseborg). The ruins of Saint Mary's Church and Korsholm Castle are now in the old town of Vaasa (Finnish: Vanha Vaasa; Swedish: Gamla Vasa).
1973 merger
Today's Korsholm municipality consists of five smaller municipalities that were merged in 1973: Korsholm in the center, Replot and Björköby in the far archipelago, Solf (Finnish: Sulva) in the southern plain and Kvevlax in the eastern plain. To reflect the new, larger municipality
