The agency won the business after a shoot-out against the Publicis network. The pitchlist initially also included the BBDO, McCann Erickson, DDB and Y&R networks.
Grey London will lead the integrated account, which includes advertising, digital and design, while G2 in Spain will manage direct marketing.
Grey will initially create a launch campaign to promote the newly created service.
OpenBank already exists in Spain and will now encompass all of Santander's European online banking services. Initially its core European markets will be Italy, Spain and the UK but Santander is expected to extend to other markets.
The process was handled by Santander's new Global Direct Banking division, which has been set up to oversee the unification of its different online banks.
Grey was not available to comment on the win.