Sites of Special Scientific Interest (SSSI) are those areas of land and water (to the seaward limits of local authority areas or MLWS) that Scottish Natural Heritage (SNH) considers to best represent our natural heritage - its diversity of plants, animals and habitats, rocks and landforms, or a combinations of such natural features. They are the essential building blocks of Scotland's protected areas for nature conservation. Many are also designated as Natura sites (Special Protection Areas or Special Areas of Conservation).The national network of SSSIs in Scotland forms part of the wider GB series.

SNH designates SSSIs under the Nature Conservation (Scotland) Act 2004. SSSIs are protected by law. It is an offence for any person to intentionally or recklessly damage the protected natural features of an SSSI. SSSIs were first designated under the National Parks and Access to the Countryside Act 1949. The majority of these were later re-notified under the Wildlife and Countryside Act 1981. All 1981 Act SSSI designations are carried forward, and all new SSSI designations are now made, under the Nature Conservation (Scotland) Act 2004.