Olavinlinna (Swedish: Olofsborg; literally St. Olaf's Castle) is a 15th-century three-tower castle located in Savonlinna, Finland. It is the northernmost medieval stone fortress still standing. The castle is built on an island in the Kyrönsalmi strait that connects the lakes Haukivesi and Pihlajavesi.
The fortress was founded by Erik Axelsson Tott in 1475 under the name Sankt Olofsborg in an effort to profit from the political turmoil following Ivan III's conquest of the Novgorod Republic. It was sited in Savonia so as to lay claim to the Russian side of the border established by the Treaty of Nöteborg.
Olofsborg withstood several sieges by the Russians during the First and Second Russian-Swedish wars. A brisk trade developed under the umbrella of the castle towards the end of the 16th century, giving birth to the town of Savonlinna, which was chartered in 1639.
Currently, the castle hosts several small exhibitions, including the Castle Museum which displays artifacts found in the castle or related to it, and the Orthodox Museum which displays icons and other religious artifacts both from Finland and Russia. The castle forms a spectacular stage for the Savonlinna Opera Festival, held annually in the summer since 1912. Olavinlinna is the initial model for Kropow Castle in the comic book King Ottokar's Sceptre, an album in the series of Adventures of Tintin created by Hergé.