A Site of Special Scientific Interest (SSSI) is a site designated as being of special interest due to the flora or fauna present or the geological make-up or physiography of the area under section 28(1) of the Wildlife and Countryside Act 1981 (WCA 1981) in England and Wales, and section 3 of the Nature Conservation (Scotland) Act 2004 in Scotland.

SSSIs are a key nature conservation designation. There are over 4,100 SSSIs in England and 1,000 in Wales, covering around 8% and 12% of the land area respectively. More than 70% of the England and Wales SSSIs (by area) are internationally important and also designated under EU legislation or international treaty obligations.