Siarö Fort, Military fort on Kyrkogårdsön island in Stockholm archipelago, Sweden.

Siarö Fort is a fortification blasted into the rock of Kyrkogårdsön, with a concrete roof four to six metres thick. Inside are command rooms, ammunition stores and quarters for about 270 soldiers, reached by a tunnel to the shore works. Two 15.2 cm m/1898 guns in single turrets came from the armoured ship HMS Svea, and two steel observation domes sit on the crest.

The decision to reinforce Vaxholm Fortress with an outer defence line was taken in 1914, and construction of Siarö Fort began in 1916 after the First World War broke out. The final inspection took place only in 1924, after the war had ended. The fort was already deemed unfit for peacetime overnight stays because of damp quarters, and by 1939 it was considered obsolete, though it remained in military use until the 1960s.

The name Siaröfortet comes from Siarö, the island where the fort was first planned before construction moved to Kyrkogårdsön, named for a cholera burial ground. Guided tours descend into the rock, and the old barracks now house a hostel and conference centre run by Antrophia Rederi AB.

Visitors can reach the fort by steamboat from Stockholm's National Museum pier or by taking bus 670 to Östanå followed by a ferry ride. Access may be limited during bad weather, so it is wise to check conditions before visiting.

The fort's underground network includes restored military facilities that recall when about 270 soldiers and sailors were stationed here. Visitors can see today how the garrison lived and worked in these tight underground spaces.