Southern Water is the private utility company responsible for the public wastewater collection and treatment in Hampshire, the Isle of Wight, West Sussex, East Sussex and Kent, and for the public water supply and distribution in approximately half of this area. Some areas within the Southern Water region are supplied by a number of smaller water supply companies. Southern Water supplies an area totalling 4,450 sq. km. and serves 2.26 million customers.

Southern Water is regulated under the Water Industry Act 1991 and since 2007 has been owned by Greensands Holdings Limited, a consortium of investors representing infrastructure investment funds, pension funds and private equity. Currently, the largest shareholders are JP Morgan Asset Management (40%), UBS Asset Management (22%), Hermes Infrastructure Funds (21%) and Whitehelm Capital (8%).

In June 2019, Ofwat proposed a fine of £126 million as a result of Southern Water’s failure to operate its wastewater treatment works properly and deliberately misreporting its performance. Ofwat found that failings had resulted in unpermitted and premature spills of wastewater from treatment works, with wastewater being released into the environment before going through the required processes.

In 2020, Southern Water pleaded guilty to 51 offences related to dumping untreated sewage into the sea and was fined £90m