South Wales (Welsh: De Cymru [ˌdeː ˌkəmri]) is a loosely defined region of Wales bordered by England to the east and mid Wales to the north. Generally considered to include the historic counties of Glamorgan and Monmouthshire, south Wales extends westwards to include Carmarthenshire and Pembrokeshire. In the western extent, from Swansea westwards, local people would probably recognise that they lived in both south Wales and west Wales. The Brecon Beacons National Park covers about a third of south Wales, containing Pen y Fan, the highest British mountain south of Cadair Idris in Snowdonia.

A point of some discussion is whether the first element of the name should be capitalised: 'south Wales' or 'South Wales'. As the name is a geographical expression rather than a specific area with well-defined borders, style guides such as those of the BBC and The Guardian use the form 'south Wales'. The Welsh Government, in their international gateway website, Wales.com, state that it should always be capitalised; however, they use the term to refer to a tourism marketing area which excludes the Swansea and Neath Port Talbot local authority areas (placing them in 'West Wales'). In contradiction, the style guide for the Welsh Government's main website, Gov.wales, uses 'south Wales'. It is always capitalised on motorway signs.

Between the Statute of Rhuddlan of 1284 and the Laws in Wales Act 1535, crown land in Wales formed the Principality of Wales. This was divided into a Principality of south Wales and a Principality of north Wales. The southern principality was made up of the counties of Ceredigion and Carmarthenshire, areas that had previously been part of the Welsh kingdom of Deheubarth ('the southern land'). The legal responsibility for this area lay in the hands of the Justiciar of South Wales based at Carmarthen. Other parts of southern Wales were in the hands of various Marcher Lords.

The Laws in Wales Acts 1542 created the Court of Great Sessions in Wales based on four legal circuits. The Brecon circuit served the counties of Brecknockshire, Radnorshire and Glamorgan while the Carmarthen circuit served Cardiganshire, Carmarthenshire and Pembrokeshire. Monmouthshire was attached to the Oxford circuit for judicial purposes. These seven southern counties were thus differentiated from the six counties of north Wales.

The Court of the Great Sessions came to an end in 1830, but the counties survived until the Local Government Act 1972 which came into operation in 1974. The creation of the county of Powys merged one northern county (Montgomeryshire) with two southern ones (Breconshire and Radnorshire).

There are thus different concepts of south Wales. Glamorgan and Monmouthshire are generally accepted by all as being in south Wales. But the status of Breconshire or Carmarthenshire, for instance, is more debatable. In the western extent, from Swansea westwards, local people might feel that they live in both south Wales and west Wales. Areas to the north of the Brecon Beacons and Black Mountains are generally considered to be in Mid Wales. The valleys and upland mountain ridges were once a very rural area noted for its river valleys and ancient forests and lauded by romantic poets such as William Wordsworth as well as poets in the Welsh language, although the interests of the latter lay more in society and culture than in the evocation of natural scenery. This natural environment changed to a considerable extent during the early Industrial Revolution when the Glamorgan and Monmouthshire valley areas were exploited for coal and iron. By the 1830s, hundreds of tons of coal were being transported by barge to ports in Cardiff and Newport. In the 1870s, coal was transported by rail transport networks to Newport Docks, at the time the largest coal exporting docks in the world, and by the 1880s coal was being exported from Barry, Vale of Glamorgan.

The Marquess of Bute, who owned much of the land north of Cardiff, built a steam railway system on his land that stretched from Cardiff into many of the South Wales Valleys where the coal was being found. Lord Bute then charged fees per ton of coal that was transported out using his railways. With coal mining and iron smelting being the main trades of south Wales, many thousands of immigrants from the Midlands, Scotland, Ireland, Cornwall, and even Italy came and set up homes and put down roots in the region. Very many came from other coal mining areas such as Somerset, the Forest of Dean in Gloucestershire and the tin mines of Cornwall such as Geevor Tin Mine, as a large but experienced and willing workforce was required. Whilst some of the migrants left, many settled and established in the south Wales Valleys between Swansea and Abergavenny as English-speaking communities with a unique identity. Industrial workers were housed in cottages and terraced houses close to the mines and foundries in which they worked. The large influx over the years caused overcrowding which led to outbreaks of Cholera, and on the social and cultural side, the near-loss of the Welsh language in the area.

The 1930s inter-war Great Depression in the United Kingdom saw the loss of almost half of the coal pits in the South Wales Coalfield, and their number declined further in the years following World War II. This number is now very low, following the UK miners' strike (1984–85), and the last 'traditional' deep-shaft mine, Tower Colliery, closed in January 2008.