A brick with a runic inscription was discovered in the 1980s during excavations of the outer bailey II of the bishop's castle of Kuusisto in SW Finland. In this article we present a transcription for the characters executed in medieval futhark and propose an interpretation for their purpose. A comparison with 27 other known medieval cases from Scandinavia shows that most of the inscribed stones come from Denmark and are mostly from ecclesiastical contexts. The interpretation of the inscription remains speculative, but it may have had an apotropaic purpose and been executed by someone involved in brick making, perhaps from southern Scandinavia. Nevertheless, the brick with the runic inscription is the first of its kind from Finland. In the Finnish context, this is the first Scandinavian language inscription on a brick from a non-urban environment.