Fort Blockhouse is a former military establishment in Gosport, Hampshire, England, and the final version of a complicated site. At its greatest extent in the 19th century, the structure was part of a set of fortifications which encircled much of Gosport. It is surrounded on three sides by water and provides the best view of the entrance to Portsmouth Harbour. It is unique in that it was built over five centuries from its original construction as a blockhouse in 1431 to the final addition of submarine base structures in the mid-1960s. Coastal fortification was abolished nationally in 1956, and the fort has not been used in a military capacity since 2021.

Gosport is a town and non-metropolitan borough, on the south coast of Hampshire, South East England. At the 2021 Census, its population was 81,952. Gosport is situated on a peninsula on the western side of Portsmouth Harbour, opposite the city of Portsmouth, to which it is linked by the Gosport Ferry. Gosport lies south-east of Fareham, to which it is linked by a Bus Rapid Transit route and the A32. Until the last quarter of the 20th century, Gosport was a major naval town associated with the defence and supply infrastructure of His Majesty's Naval Base (HMNB) Portsmouth. As such over the years extensive fortifications were created.