Iris will now work as the lead digital agency for the petrol giant, and will initially produce campaigns for its retail, fuel and lubricants accounts.
The agency's brief will include web development, online advertising and electronic customer relationship marketing.
Iris already works with Shell on cross-promotional activity. The incumbent lead agency, Digitas London, continues to work with Shell's corporate department, and is currently redesigning the Shell.com website.
In May, Iris hired Drew Thomson, the former chief executive of Airmiles, as its chairman to spearhead the agency's expansion plans across Europe.
One of his first initiatives was to create a through-the-line strategy agency, jointly owned by Rise Communications, called iRise.
Both Shell and Iris declined to comment.