A cay or key is a small, low island consisting mostly of sand or coral. The word is used almost exclusively in the West Indies, though it is sometimes used in the context of other tropical environments, such as the Great Barrier Reef and especially in the Florida Keys.

Sand cays are formed when tidal action, wind and sea birds deposits, over a long period ot time, coral debris and sand on to reef flats, usually on the leeward side of the reef, but occasionally on the windward side. The weather also affects the formation of cays greatly; large tides would bring much more debris onto the cay and thus make it larger, whilst a hurricane could completely obliterate a cay.

The Cay is the title of a book by Theodore Taylor.