The Giant's Castle is a promontory fort of the Iron Age, on St Mary's, Isles of Scilly, England.

The rocky promontory, area 0.4 hectares (1 acre), is defended by sea cliffs, 15 to 25 metres (49 to 82 ft) high, on the east, south and south-west sides.

To the north there are three banks and ditches, 8 to 12 metres (26 to 39 ft) apart; these occupy most of the steep northern slope linking the promontory to the mainland.

[1][2] During the Second World War, a practice firing target for military aircraft was made in the western end of the outermost rampart.

Fragments of early Iron Age pottery were recovered from cuttings made by the army.