Merlin's Cave is the name given to a cave located beneath Tintagel Castle, 5 kilometres south west of Boscastle, Cornwall. It is 100 metres long, passing completely through Tintagel Island from Tinagel Haven on the east to West Cove on the west. It is a sea cave formed by marine erosion along a thrust plane between slate and volcanic rocks. Tennyson made Merlin's Cave famous in his Idylls of the King, describing waves bringing the infant Arthur to the shore and Merlin carrying him to safety.
The lower portion of the steps down to the cave have been washed away, and it is necessary to clamber down some boulders to reach the beach. The cave fills with water at high tide, but has a sandy floor and is explorable at low tide.