A crannog (; Irish: crannóg [ˈkɾˠan̪ˠoːɡ]; Scottish Gaelic: crannag [ˈkʰɾan̪ˠak]) is typically a partially or entirely artificial island, usually built in lakes and estuarine waters of Scotland, Wales, and Ireland. Unlike the prehistoric pile dwellings around the Alps, which were built on the shores and not inundated until later, crannogs were built in the water, thus forming artificial islands.

Crannogs were used as dwellings over five millennia, from the European Neolithic Period to as late as the 17th/early 18th century. In Scotland there is no convincing evidence in the archaeological record of Early and Middle Bronze Age or Norse Period use. The radiocarbon dating obtained from key sites such as Oakbank and Redcastle indicates at a 95.4 per cent confidence level that they date to the Late Bronze Age to Early Iron Age. The date ranges fall after around 800 BC and so could be considered Late Bronze Age by only the narrowest of margins.

Crannogs have been variously interpreted as free-standing wooden structures, as at Loch Tay, although more commonly they are composed of brush, stone or timber mounds that can be revetted with timber piles. However, in areas such as the Outer Hebrides of Scotland, timber was unavailable from the Neolithic era onwards. As a result, crannogs made completely of stone and supporting drystone architecture are common there. Today, crannogs typically appear as small, circular islets, often 10 to 30 metres (30 to 100 ft) in diameter, covered in dense vegetation due to their inaccessibility to grazing livestock.