Euro RSCG 4D will be responsible for all of IBM's online branding, including the development of strategy, technology, design and social media outreach.
The appointment extends Euro RSCG's existing relationship with IBM as the agency had already managed the tech giant's email and relationship marketing for several years.
Euro will work with IBM's creative and media agencies — WPP's Ogilvy & Mather and Mindshare.
John Kennedy, vice president of corporate marketing at IBM, said: "Euro has deep digital expertise, a broad global footprint and a strong track record working with IBM on email and relationship marketing.
"Our current agency partners are teaming with IBM to do great digital work in advertising, demand generation and events, and we are confident that adding Euro to the mix in a lead role will further enhance our digital capabilities and presence."
The size of the account was not disclosed, but IBM's digital budget comes out of a reported US$1.25bn global ad spend.
IBM's digital business win is Euro's second in as many weeks for holding company Havas. Its media agency MPG won the estimated £150m global media planning and buying account for Hyundai Kia last week.