The agency won the business after a four-way pitch against other RBS roster agencies.
The Communications Agency, which has worked on campaigns for RBS Group’s retail and corporate divisions since 2005, will handle integrated advertising for the business banking side of both NatWest and RBS.
The agency will be tasked with creating advertising, digital and direct activity to target businesses from start-ups to large corporate customers of the banks.
Adam Leigh, the chief executive of The Communications Agency, said: "We’re delighted to have been given such a big vote of confidence in our creative and strategic thinking. We are very excited about the direction we will be taking in 2013 to create work that is competitively distinctive."
The review was handled by RBS Group’s procurement department and kicked off in November last year.
M&C Saatchi continues to handle the bulk of RBS’s retail banking business. Other agencies on the finance group’s roster include CHI & Partners, CST The Gate and TBWA\Manchester.
Separately, RBS Group is reviewing its digital creative agency roster for its main consumer banking brands.