Sisu was a Finnish state-owned icebreaker. Built in 1939 at Wärtsilä Hietalahti Shipyard in Helsinki, she was one of the world's first diesel-electric icebreakers. In addition to icebreaking duties, she served as a submarine tender for the Finnish Navy during the summer months until the end of the Continuation War.

An icebreaker is a special-purpose ship or boat designed to move and navigate through ice-covered waters, and provide safe waterways for other boats and ships. Although the term usually refers to ice-breaking ships, it may also refer to smaller vessels, such as the icebreaking boats that were once used on the canals of the United Kingdom.