BBVA, one of Spain's largest banks, is moving desktop applications to Google Apps' cloud computing systems.
The bank told the BBC it was moving 110,000 staff over to the systems, including UK staff in the City of London, but insisted it would keep client data separate.
Traditionally, banks have been among the last companies to say they will consider cloud adoption, and are particularly nervous around who controls sensitive customer data.
Carmen Herranz, director of innovation at BBVA, said the company's customer data would "stay in our own data centres", while email, calendar, documents and chat applications would be farmed out to the cloud.
The rollout will be complete by the end of the year. "The main goal is to promote innovation and making decisions and increase productivity. We are in a challenging market and need to make faster and more accurate decisions," she said, adding that mobile access to systems was also vital.