The following is a list of Sites of Special Scientific Interest (SSSI) by area of search, in the United Kingdom. SSSIs are areas of conservation, consisting of protected areas, recognised for their biological or geological significance. In Northern Ireland an SSSI is called an area of special scientific interest (ASSI).
The English counties were revised under the 1974 reorganisation of local government. Until the 2010s, Natural England, which maintains the database of English SSSIs, kept the listing of counties as it was in 1974, but by 2015 they had updated their lists to reflect some later changes. However, two counties abolished in 1996 are still used as areas of search by Natural England, Avon (which is now divided between Somerset, Gloucestershire and Bristol) and Cleveland (which is now divided into County Durham and North Yorkshire).
- A A number of sites are split between more than one county and will be included in this table more than once.
|County Antrim||TBC||TBC||TBC||TBC||List of ASSIs in Antrim|
|County Armagh||TBC||TBC||TBC||TBC||List of ASSIs in Armagh|
|County Down||TBC||TBC||TBC||TBC||List of ASSIs in Down|
|County Fermanagh||TBC||TBC||TBC||TBC||List of ASSIs in Fermanagh|
|County Londonderry||TBC||TBC||TBC||TBC||List of ASSIs in Londonderry|
|County Tyrone||TBC||TBC||TBC||TBC||List of ASSIs in Tyrone|
|Area of Search||Number of sites||Sites list|
|Brecknockshire||TBC||TBC||TBC||84||List of SSSIs in Brecknock|
|Carmarthen & Dinefwr||TBC||TBC||TBC||79||List of SSSIs in Carmarthen & Dinefwr|
|Ceredigion||TBC||TBC||TBC||100||List of SSSIs in Ceredigion|
|Clwyd||TBC||TBC||TBC||81||List of SSSIs in Clwyd|
|East Gwynedd||TBC||TBC||TBC||103||List of SSSIs in East Gwynedd|
|Gwent||TBC||TBC||TBC||78||List of SSSIs in Gwent|
|Mid & South Glamorgan||TBC||TBC||TBC||69||List of SSSIs in Mid & South Glamorgan|
|Montgomeryshire||TBC||TBC||TBC||76||List of SSSIs in Montgomery|
|Preseli & South Pembrokeshire||TBC||TBC||TBC||75||List of SSSIs in Preseli & South Pembrokeshire|
|Radnorshire||TBC||TBC||TBC||87||List of SSSIs in Radnor|
|West Glamorgan||TBC||TBC||TBC||56||List of SSSIs in West Glamorgan|
|West Gwynedd||TBC||TBC||TBC||135||List of SSSIs in West Gwynedd|