Hoy (from NorseHáey meaning high island) is an island in Orkney, Scotland. With an area of 143 square kilometres (55 sq mi) it is the second largest in the archipelago after the Mainland. It is connected by a causeway called The Ayre to South Walls. Unusually, the two islands are treated as one entity by the UK census.
The dramatic coastline of Hoy greets visitors travelling to Orkney by ferry from the Scottish mainland. It has extremes of many kinds: some of the highest sea cliffs in the UK at St John's Head, which reach 350 metres (1,150 ft); the impressive and famous sea stack, the Old Man of Hoy; some of the most northerly surviving natural woodland in the British Isles and the remote possibility that the Orkney charr (Salvelinus inframundus), last described in 1908, survive in Heldale Water. The most northerly Martello Towers were built to defend the area during the Napoleonic War, but were never used in combat.
The highest point in Orkney, Ward Hill, is on Hoy.
The main naval base for the British fleet Scapa Flow in both the First and Second World Wars was situated at Lyness in the south-east of the island. Some rather incongruous Art Deco structures nearby date from this period.
An unusual rock-cut tomb, the Dwarfie Stane, lies in the Rackwick valley in the north of the island. It is unique in northern Europe, bearing similarity to Neolithic or Bronze Age tombs around the Mediterranean. The tomb gets its name as it is very small and was said to be carved by dwarves.