Spanish DDB agency C14torce is understood to be charged with launching cars in new categories. Adam & Eve/DDB will offer network support in the UK. Lola MullenLowe was the incumbent.
Droga5 will focus on "specific projects for next year", according to a Seat spokeswoman.
The move is part of Seat’s strategy to work with a global network agency and a "creative challenger".
A source said the strategic importance of Seat has increased for VW Group after the flagship brand suffered from the emissions scandal.
The Seat spokeswoman added: "We are taking on the biggest product offensive in our history that will enable us to strengthen the brand and boost sales."