"We are delighted to be making this strategic investment -- a majority stake in Qinyuan -- our biggest acquisition in China for more than 10 years," Unilever chief executive Paul Polman said in a statement.
Rotterdam-based Unilever, which last year clocked up a net profit of 4.84 billion euros (USD 6.56 billion), declined to put an amount on the deal or disclose the size of its stake.
Qinyuan last year made almost 140 million euros in sales in China's rapidly-expanding water purification market, which has grown more than 20 per cent a year over the last three years, Unilever said.
Founded in 1930, Unilever employs some 174,000 people worldwide and has some of the world's best-known brands like Knorr, Omo, Lipton and Magnum ice cream in its portfolio.
Qinyuan is a high-tech company that makes water purifiers, drinking water equipment and water treating membranes and has around 2,500 employees. It was founded in 1998.