Thorne Island is a rocky islet and part of the community of Angle, Pembrokeshire, Wales, with an area of 2 acres (8,100 m2), dominated by a coastal artillery fort built to defend the Milford Haven Waterway in the mid-19th century.

Thorne Island commands the entrance to the anchorage of Milford Haven and access to the former Royal Dockyard at Pembroke Dock.

This was due to the limited number and size of guns that the existing fort could bring to bear on any fast moving target.

[3] The report recommended to Lord Palmerston that further forts should be strengthened or constructed to counter the potential threat of the French Navy.

Divers recognise over twelve wrecks that are worth diving in the area of the island, but of special interest is the 1878 built sailing ship Loch Shiel that sank in 1894.