Carat UK has extended its 20-year relationship with Santander, following an international six-month pitching process, aiming to consolidate its media into a single agency across Europe.
Carat has also acquired the media duties for Spain and Portugal, previously handled by Havas Media's Arena.
Fellow dentsu agency, Dentsu X, has secured the account in Poland and the US.
The dentsu offering saw off tough competition from an unnamed IPG agency and a Havas Media Group agency during the final stages of the process.
Clare Chapman, chief executive at Carat UK, said she was "proud" Santander had chosen to partner with Carat and build on their relationship.
She added: "I am really pleased that the strong foundations we have built in the UK have been mirrored across regions by dentsu, where we have consistently shown the benefit of our integrated network and talent when it comes to meeting Santander's mission to help people and business prosper."