The appointment, which is worth an estimated £50m, will see CHI produce a pan-European advertising campaign to support the January launch of the new Yaris, Toyota's small car competitor for the Renault Clio and the Ford Fiesta.
Last year, CHI won the £8m business to launch the youth-targeted Aygo across Europe. In August this year, the agency was appointed to the £35m Corolla account, which saw it develop a television and print campaign that ran across all of Toyota's 25 markets in Europe.
As with CHI's work on the Aygo and Corolla, its work for the Yaris is not expected automatically to run in the UK. With the other marques it handles, CHI creates an idea, which is then offered as an option to all of Toyota's European markets.
The move will be a blow to Saatchis' network. Toyota is its second-biggest account behind Procter & Gamble, spending $2.5bn globally last year.
CHI declined to return calls, while Toyota was unavailable for comment.
If you have an opinion on this or any other issue raised on Brand Republic, join the debate in the Forum.