The Southern Region was a region of British Railways from 1948 until 1992 when railways were re-privatised. The region ceased to be an operating unit in its own right in the 1980s. The region covered south London, southern England and the south coast, including the busy commuter belt areas of Kent, Sussex and Surrey. The region was largely based upon the former Southern Railway area.

The Southern Railway was still comparatively profit-making despite World War II, thanks to its extensive third rail DC electrification and the intensive service patterns this allowed for. However, large-scale investment was required in the infrastructure of all of the "Big 4" companies, including the Southern.

The Transport Act 1947 provided for the nationalisation of all heavy rail systems in the UK to allow for this investment and, in theory, to improve the rights of railway workers. The railway companies were amalgamated into British Railways, part of the British Transport Commission, and six geographic and administrative regions were created out of the previous four companies. The Southern Railway, being relatively self-contained and operated largely by electric traction, was incorporated almost intact as the new Southern Region. The acting General Manager of the Southern Railway, John Elliot became the first Chief Regional Officer of the Southern Region.[1]

The Southern Region also inherited some independent light railways, namely the East Kent Light Railway, the Kent and East Sussex Railway and the North Devon and Cornwall Junction Light Railway.

The Southern Region served southern London, Kent, Surrey, Sussex, Hampshire, the Isle of Wight, and some areas of Dorset, Wiltshire and Berkshire. There was also an unelectrified service to parts of Devon and Cornwall, deep in what was largely Western Region territory, known colloquially as "The Withered Arm". The Southern Region also assumed operating responsibility for the Somerset & Dorset Joint Railway (although the provision of motive power fell to the London Midland Region). There were three operating divisions: Eastern, Central and Western which correspond approximately to the three current franchise areas.