Castle Cornet is a large island castle in Guernsey, and former tidal island, also known as Cornet Rock or Castle Rock. Its importance was as a defence not only of the island, but of the roadstead. In 1859, it became part of one of the breakwaters of Guernsey's main harbour, St Peter Port harbour.

The island measures about 2 hectares (4.9 acres) in area, with a length of 175 metres (574 ft) and a width of 130 metres (430 ft). It lies not quite 600 metres (2,000 ft) east of the coast of Guernsey.

Formerly a tidal island, like Lihou on the west coast of Guernsey, it was first fortified as a castle between 1206 and 1256, following the division of the Duchy of Normandy in 1204. The wardenship of Geoffrey de Lucy (1225–26), has been identified as a time of fortification in the Channel Islands: timber and lead was sent from England for castle building in Guernsey and Jersey. At that time the structure consisted of a keep, a chapel, two courtyards, and curtain walls. In 1338, when a French force captured the island, they besieged Cornet, capturing it on 8 September; the French then massacred the garrison of eleven men at arms and 50 archers. The island was retaken in 1340, and the castle was recaptured in August 1345, after a three-day attack by professional soldiers and the local militia. The French had spent their seven-year occupation improving the defences, including probably the barbican. In 1358, the French returned and again captured the castle, but they were evicted the following year and an island traitor was executed.

In 1372, Owain Lawgoch, a claimant to the Welsh throne, leading a free company on behalf of France, attacked Guernsey in an assault popularly called "La Descente des Aragousais". Owain Lawgoch withdrew after killing 400 of the island militia, but without capturing the besieged Castle Cornet, which he found strong and well supplied with artillery. In yet another assault the French again captured the castle in 1380, before island forces again evicted them a short time later. In the early fifteenth century improvements were made: the Carey tower was constructed around 1435. A French assault in 1461, was repulsed.

The construction costs for works, repairs, maintenance, and the garrison were met from revenues raised in the island by the Warden (sometimes called "Keeper of the Castle") under royal warrant.

The advent of cannon and gunpowder led to the castle being remodelled (1545 and 1548). In 1547, the French, having captured Sark, descended on Guernsey; they met with gunfire from off St Peter Port and bombardment from cannon at the Castle. Additional building works took place. Prof. John Le Patourel mentions in The Building of Castle Cornet that in 1566, iron and hammers were taken to "Creavissham" (Crevichon), and that the island was quarried for materials for the castle. Sand was brought from Herm. In 1594, the "Royal Battery" was completed, as was the Sutlers house, and bastions of improved, polygonal form were constructed.

Sir Walter St John drowned whilst staying at the castle in August 1597.

In 1627, King Charles I reduced the Crown's cost of running Castle Cornet by granting additional rights to Guernsey in a charter, in return for which the island became responsible for supplying victuals to the castle, including annual amounts of 100 tuns (1 tun holds 252 gallons) of beer, 600 flitches of bacon, 1,200 pounds of butter, 20 whey (around 4,600 pounds) of cheese, 3,000 stockfish, 300 pounds of tallow, twelve bulls, wood and coal.