Rothesay Castle is a ruined castle in Rothesay, the principal town on the Isle of Bute, in western Scotland. Located at NS086646, the castle has been described as "one of the most remarkable in Scotland", for its long history dating back to the beginning of the 13th century, and its unusual circular plan.

Rothesay is the principal town on the Isle of Bute, in the council area of Argyll and Bute, Scotland. It lies along the coast of the Firth of Clyde. It can be reached by ferry from Wemyss Bay, which offers an onward rail link to Glasgow. At the centre of the town is the 13th-century ruin Rothesay Castle, unique in Scotland for its circular plan.