The Diocese of Åbo was the medieval, pre-Reformation Catholic predecessor of the later Archdiocese of Turku, a Evangelical Lutheran Church of Finland jurisdiction.
TO ELABORATE It was probably founded in the 12th century, but the Bishop of Finland is first mentioned only in 1209. Diocese seems to have been independent of secular powers until the so-called Second Swedish Crusade in 1249. The diocese was renamed as the Diocese of Turku by 1259 at the latest, by which time it had been assimilated to the set of Swedish dioceses.
The see was located in Nousiainen by 1234, when Bishop Thomas signed a letter there. The bishop had received papal approval for the transfer of the see in 1229, but it is not known if the transfer actually took place. If the see had been located somewhere else before, the location is unknown. The see was later moved to Koroinen close to the current cathedral of Turku, probably soon after the Swedish conquest in 1249.