The new model is expected to launch in 2005 and will be aimed at drivers between 18 and 35 years old. The pitch was run from Toyota's European office in Belgium. No details about the model have yet been disclosed and Toyota would not confirm the win.
CHI won the business after a pitch against Attik, an agency based in Hull. Attik's San Francisco counterpart won the brief to launch the Toyota Scion in the US last year, weakening Saatchis' grip on the business in the US market.
In 2002, Toyota called a pitch for the launch of the Avensis, which Saatchis won.
Toyota is one of Saatchis' biggest global clients, after Procter & Gamble.
It is the world's fourth-biggest advertiser, spending $2.5 billion globally.
Saatchis and CHI were unavailable for comment as Campaign went to press.