Thames Water Utilities Limited, trading as Thames Water, is a British private utility company responsible for the water supply and waste water treatment in most of Greater London, Luton, the Thames Valley, Surrey, Gloucestershire, north Wiltshire, far west Kent, and some other parts of England. Like other water companies, it has a monopoly in the regions it serves.

With origins dating back to the formation of the New River Company in 1609, Thames Water was established in 1989 during privatisation of the water industry in England and Wales. The name of the company reflects its role serving the drainage basin of the River Thames; water is sourced from the Thames as well as a number of other rivers and boreholes.

The UK's largest water and wastewater services company, Thames Water is responsible for an extensive water management infrastructure which includes the Thames Water Ring Main around London, one of Europe's largest wastewater treatment works and the UK's first large-scale desalination plant—both at Beckton in east London—and the £4.2 billion Thames Tideway sewer (which went into service in 2025). Per day, the company supplies 2.5 billion litres (550 million imperial gallons) of drinking water and treats 4.6 billion litres (1,000 million imperial gallons) of wastewater. It serves a population of 15.5 million people—over a quarter of England's population—but its ageing infrastructure is prone to leakage and is a frequent cause of pollution, for which it has been repeatedly prosecuted and fined.

Current shareholders include four major pension funds and four overseas investment funds which between them hold over 90% of the company's shares. The company has been criticised for paying substantial dividends to shareholders while simultaneously taking out loans, accumulating over £16 billion in debts. From June 2023, Thames Water was repeatedly said to be close to financial collapse. In April 2024, the UK Government was reported to be considering plans to temporarily renationalise the company (putting it into a special administration regime, SAR), and in January 2025 began talks with potential special administrators. A £3bn emergency bailout was agreed in March 2025, giving Thames more time to repair its finances, but in June 2025 the government stepped up preparations for temporary nationalisation of the company.

Thames Water can trace its history back to the construction of the New River, which was started in 1604 by Edmund Colthurst to carry fresh water from Hertfordshire into London. The business of the New River was taken over by the New River Company, officially founded by royal charter in 1619, under the leadership of Hugh Myddelton. Although earlier small-scale water supply operations existed, the New River Company was the first water supply company and is the earliest direct ancestor of Thames Water today.

During the 1850s, John Snow and William Farr's identification of the 1854 Broad Street cholera outbreak provided a stimulus for the better treatment of sewage. The Thames Conservancy was established in 1857 with unified control over water supply, drainage and navigation. The Great Stink occurred in 1858, and focused government and public opinion on cleaning up the Thames. To resolve these issues, the Metropolitan Board of Works, under the leadership of Chief Engineer Joseph Bazalgette, constructed a large network of sewers by 1870, many of which are still in use today.

In 1904 the New River Company and eight other water companies serving London were taken into public ownership under the control of the newly-founded Metropolitan Water Board. In 1973 the responsibility for water supply and sewage in the Thames catchment was transferred to the Thames Water Authority.

In 1989, the responsibility for navigation, regulatory, river and channels management was transferred from the Thames Water Authority to the National Rivers Authority, which became part of the Environment Agency in 1996. The remainder of the Thames Water Authority was privatised as Thames Water Utilities Limited.