ZenithOptimedia is expected to take on the media planning and buying.
No budget has been set for the work, which will run only if depleted water supplies lead to rationing and hosepipe bans. The brief also includes opportunities to work on foreign projects for the utility company, which is part of the German conglomerate RWE.
Thames Water, previously with Bates UK, chose not to automatically follow the majority of Bates clients into the WPP-owned J. Walter Thompson after WPP's takeover of Bates' Cordiant parent. Instead, it invited JWT and M&C Saatchi to pitch for the business.
Mike Moran, Thames Water's worldwide director of marketing and strategy, cited M&C Saatchi's creative and strategic thinking and its "strong ability to provide concepts which can be used across a number of media, not just advertising" as the reason for the appointment.
Thames Water, is the world's third-largest water company, serving 70 million customers in more than 20 countries. This includes 13 million people across 5,000 square miles of London and the Thames Valley.
The company is currently in talks with the Chinese Government to buy a 50 per cent stake in the state-owned company supplying water to four million people in and around Shanghai.
Nick Hurrell, M&C Saatchi's joint chief executive, said: "Thames Water is ambitious and forward-looking and is looking for world-class communications to match."