The Riihisaari Museum, also known as the Savonlinna Provincial Museum, occupies a former warehouse on a small island linked by footbridge to the mainland next to the castle. It covers the history of the Savonlinna region and the wider Lake Saimaa system — the boat-building, log-floating, and steamship traffic that defined the area until the railways arrived in the 20th century.
The outdoor exhibits include three preserved historic ships moored at the dock: the Salama, the Mikko, and the Savonlinna. The museum also has a small but well-curated section on the Saimaa ringed seal, the freshwater seal that survives only in this lake system. Open daily in summer, weekends only in winter.