The agency will focus on RBS's retail banking unit and is briefed to handle all of the creative tasks for the finance company's high-street banks.
RBS is the second-biggest bank in Scotland, behind The Bank of Scotland. It has 640 branches - 340 in Scotland and 300 in the rest of the UK. It services 3.5 million customers.
The win sees M&C Saatchi extend its relationship with RBS. It already works for its NatWest, Direct Line and Privilege Insurance brands.
A spokesman for RBS said: "Following a review of our agency support last year we have decided to consolidate our retail banking unit into M&C Saatchi. The agency is already our appointed agency for the RBS Group brand and NatWest retail banking."