The Raseborg Castle (Finnish: Raaseporin linna, Swedish: Raseborgs slott), is a medieval castle in Raseborg, Finland. The castle was founded by Bo Jonsson Grip and it is thought that the castle's first phase was completed sometime between 1373 and 1378. The first written data about the castle is from 1378. Its main purpose was to protect Sweden's interests in southern Finland against the Hanseatic city of Tallinn. The castle was originally built on a small island in the north end of a sea bay.