St Michael's Mount (Cornish: Karrek Loes y'n Koes meaning "grey rock in the woods") is a tidal island located 366 m (400 yd) off the Mount's Bay coast of Cornwall, England, United Kingdom. It is a civil parish and is united with the town of Marazion by a man-made causeway of granite setts, passable between mid-tide and low water.

The island exhibits a combination of slate and granite (see Geology below). Its Cornish language name-—literally, "the grey rock in the wood"-—may represent a folk memory of a time before Mount's Bay was flooded. Certainly, the Cornish name would be an accurate description of the Mount set in woodland. Remains of trees have been seen at low tides following storms on the beach at Perranuthnoe, but radiocarbon dating established the submerging of the hazel wood at about 1700 BC. The chronicler John of Worcester relates under the year 1099 that St. Michael's Mount was located five or six miles (10 km) from the sea, enclosed in a thick wood, but that on the third day of the nones of November the sea overflowed the land, destroying many towns and drowning many people as well as innumerable oxen and sheep; the Anglo-Saxon Chronicle records under the date 11 November 1099, "The sea-flood sprung up to such a height, and did so much harm, as no man remembered that it ever did before". The Cornish legend of Lyonesse, an ancient kingdom said to have extended from Penwith toward the Isles of Scilly, also talks of land being inundated by the sea.