Fort Darnet is a nineteenth-century military installation on the River Medway in Kent, England, that formed part of the defences of Chatham Naval Dockyard. Fort Darnet, like its twin Hoo Fort 1 km (0.6 mi) upstream, was built on the recommendations of the 1859 Royal Commission on an island covering Pinup Reach, the inner navigable channel of the River Medway.

Building started on the island in 1870 and then finished in 1872. The forts were originally designed for two tiers of guns mounted in a circle, with a boom strung between them, but there were many problems with subsidence. After extensive cost overruns they were completed in 1872 with one tier of eleven guns: a mixture of eight 9-inch and three 7-inch rifled muzzle-loaders. The boom was not implemented, though there were plans to mine the channel if thought needed. Fort Darnet was originally designed for a garrison of up to 100 men.

The fort was used for gunnery practice until one of the guns cracked in its casemate, as reported in ‘The Chatham Observer’ on 25 January 1879.

The forts were never used in anger, and were decommissioned before the First World War. In the Second World War the fort was used as an observation post, with platforms and pillboxes built on top. The fort is still in fair condition, however the magazine level is flooded. The island can be freely visited by boat, though the landing is muddy.

Up to the 1980s, the island was used for picnicking and other leisure pursuits.

It is scheduled under the Ancient Monuments and Archaeological Areas Act 1979.

• Crowdy, R, Medway's Island Forts, (1979)
• Gulvin, K R, The Medway Forts, (1976), 18-19
• Smith, V T C, Strategic Study of Kents Defences - Fort Darnet, (1999)

Media related to Darnet Fort, Medway at Wikimedia Commons