Fort of Drottningskär, Military fortress and museum in Karlskrona, Sweden.

The Fort of Drottningskär is a water fortress built on a small island just outside Karlskrona, in southern Sweden, featuring four bastions and a central donjon fitted with naval artillery. The entire structure sits in open water and once formed the outer defensive ring of the Swedish naval base.

Construction began in 1680 as part of the new Karlskrona naval base, after Karl XI moved the Swedish fleet to Blekinge; Erik Dahlbergh drew the plans. In the 1730s walls, a moat and a ravelin were added on Aspö to guard the landward side. Drottningskär was the main fort of Karlskrona's outer defence line until the 1820s, was disarmed in the 1870s and left the defence organisation in 1895.

The four bastions carry the names of Swedish queens: Kristina, Hedvig, Maria and Ulrika. The fort is a state-listed building monument since 25 January 1935 and today serves as a well-preserved museum site, with wooden bridges reconstructed in 2008 linking the donjon to the landward outworks.

The fort is only accessible by boat from Karlskrona harbor, and visits are typically seasonal and run as guided tours. Since the site is surrounded by water, it is worth checking opening dates in advance and bringing a windproof layer.

In her diary, Catherine II called Drottningskär "the gray louse" that ruined her plans, after her fleet declined to run the sound in 1788–1790. The nickname refers to the fort's small, stubborn presence at the inlet to Karlskrona.