China Investment Corporation (CIC), the country's $410bn sovereign wealth fund, has bought 8.68% of the company behind UK utility group Thames Water.
It marks the fund's first acquisition in the UK, and follows a visit to China this week by Chancellor George Osborne.
Thames Water is owned by Kemble Water, a consortium of investors led by Australian bank Macquarie.
CIC was set up in 2007 to invest some of China's huge $3.18 trillion in foreign exchange reserves.
In a one-sentence statement on its website, CIC said it bought the stake through a wholly-owned subsidiary. No purchase price was disclosed.
Thames Water, acquired by Kemble in 2006, is the UK's largest water and sewerage company, serving about 14 million customers.
CIC chairman Lou Jiwei had said recently that his company was interested in investing in European and US infrastructure.
Mr Osborne met China's Vice-Premier Wang Qishan and the heads of CIC and the ICBC bank on Tuesday.
The chancellor also announced that Britain had agreed to work with Hong Kong to develop the City of London as a major offshore centre of trade in China's yuan currency.
In December, the Abu Dhabi Investment Authority, another huge sovereign wealth fund, bought 9.9% of Kemble for an undisclosed price.