Stuart Siddall, who has been the company's finance chief since 2011, but who has been in CFO roles since 1990, will retire at the end of the year.
The company said it will begin the search for a replacement once a new chief executive has been named. The company announced in November that chief executive Martin Baggs was stepping down - and this morning it said a replacement for Baggs will be named "shortly".
Meanwhile, Thames Water said Nick Fincham, currently its strategy and regulation director, will be added to the executive board.
Sir Peter Mason, its chairman, thanked Siddall, saying he had played a "major role in increasing the resources directed to front-line services".
“Stuart has always made it clear that he would want to stand down from full-time roles about now and we wish him well in his retirement. Nick Fincham has been a highly effective member of the executive team, including leading the way in the last regulatory price review, and we are delighted to welcome him to the board.”