Southern is the brand name used by the Govia Thameslink Railway (GTR) train operating company on the Southern routes of the Thameslink, Southern and Great Northern franchise in England. It is a subsidiary of Govia, a joint venture between transport groups Go-Ahead and Keolis, and has operated the South Central rail franchise since August 2001 and the Gatwick Express service since June 2008. When the franchise was subsumed into GTR, Southern was split from Gatwick Express and the two became separate brands, alongside the Thameslink and Great Northern brands.

Southern operates the majority of commuter services from its Central London terminals at London Bridge and London Victoria to South London, East and West Sussex, as well as regional services in parts of Hampshire, Kent and Surrey. It also provides services between Milton Keynes and Croydon via the West London Line.

Major destinations served include Beckenham Junction, Bexhill, Bognor Regis, Brighton, Caterham, Chichester, Crawley, Eastbourne, East Croydon, East Grinstead, Epsom, Epsom Downs, Gatwick Airport, Horsham, Mitcham Junction, Leatherhead, Littlehampton, Portsmouth Harbour, Redhill, Southampton Central, Sutton (London), Tattenham Corner, Uckfield and Worthing.

It also operates services from Eastbourne to Ashford, Brighton to Ore, Brighton to Seaford, Brighton to Southampton and Clapham Junction to Milton Keynes Central.

Southern had the most dissatisfied passengers in Britain for three consecutive years, according to the national rail passenger surveys in 2016, 2017 and 2018. In 2018 it scored poorly for reliability, punctuality and value for money.