The move climaxes a period of ardent wooing of Seat by Grey in the wake of the Cordiant group's acquisition by Sir Martin Sorrell's WPP. The takeover threw up an immediate conflict with Ford, WPP's biggest single client.
Ed Meyer, Grey's chairman, and Bernd Michael, the network's European chief, made Seat's capture a top priority. Grey has previously handled VW's Skoda business, while MediaCom, Grey's media independent, has VW business in a number of countries.
Seat was previously run out of the Cordiant-owned Bates office in Barcelona and key members of the Bates team will follow the business into Grey.
Lutz Kothe, Seat's head of marketing communication, said: "Since we are very much interested in keeping the positive momentum of Seat, a well-organised transition to the new agency is a key issue."