National nature reserves in Wales are selected and designated by Natural Resources Wales (NRW). There are 76 reserves all of which are also SSSIs, they cover 26,536 hectares (265.36 km2), or less than 1.5% of the land area of Wales. They range in size from Dan yr Ogof at 0.5 hectares (5,000 m2) to Berwyn which covers 7,920 hectares (79.2 km2). A national nature reserve is designated because of its special biological, geological, habitat or landscape value and most in Wales are open to the public. They include upland and lowland areas, several offshore islands and in some cases estuarine and intertidal zones.

Dinefwr Park National Nature Reserve is an 800 acres (320 ha) estate about a mile from the centre of Llandeilo in Carmarthenshire owned by the National Trust, with a mansion in the centre. The highest point is occupied by the ruined Dinefwr Castle, ancient castle of the Deheubarth kingdom. It is a grade I Historic Park and a Site of Special Scientific Interest.