Rothesay Castle is a ruined castle in Rothesay, the principal town on the Isle of Bute, Scotland. 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.

The castle comprises a huge curtain wall, strengthened by four round towers, together with a 16th-century forework, the whole surrounded by a broad moat. Built by the Stewart family, it survived Norse attacks to become a royal residence. Though falling into ruin after the 17th century, the castle was repaired by the Marquess of Bute before passing into state care in the 20th century.