Operation Tanne Ost was a German operation during World War II to capture the island Suursaari in the Gulf of Finland before it could fall into Soviet hands. Suursaari was especially important because it worked as a lock in the Finnish gulf guarding the minefields keeping the Soviet Baltic Navy in Kronstadt.

The operation was initially planned with another operation to capture the Åland Islands ("Tanne West"), which was not carried out.

On September 15, a first wave of 1400 men from both the Wehrmacht and the Kriegsmarine were loaded on ships in Tallinn. Before the assault the German commander tried to negotiate with the Finnish commander on Suursaari, as he thought the finns might leave without resistance. The negotiations were a complete failure and the invasion had to begin. When the German ships approached Suursaari the Finnish crew on the island opened fire. However most of the troops made it to the beaches, but there the difficulties continued, somewhat because of the Finnish defense being much more numerous than expected. After sunrise the Soviets made bombing runs on the German ships. A second wave of untrained Kriegsmarine troops was withdrawn before they could land. The operation ended in a complete failure, with the Finns capturing some 700 German prisoners.

This operation marked the beginning of hostility between German and Finnish troops, known as the Lapland War. Before this, the Germans had conducted their withdrawal from Northern Finland without incident.

See also: List of military operations