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 $3.18tn (£2tn) 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.
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