ICrossing, which acquired Spannerworks for more than £10m in February 2007, said the integration combines the two companies' services in search marketing, social media and web development, allowing it to better serve its growing client base.
Arjo Ghosh, founder and chief executive of Spannerworks, has been named chief executive of iCrossing UK and will report to Don Scales, president and chief operating officer of iCrossing.
Ghosh will be based in Brighton, leading the company's corporate development and UK operations.
Jeffrey Herzog, founder and global chief executive of iCrossing, said: "For the past year, Spannerworks and iCrossing have been operating as one company, with a shared belief that search is the common pathway to all digital marketing.
"One brand, supported by this common belief, will fortify iCrossing's position in the global marketplace and strengthen our ability to deliver global digital marketing programs to our clients."
Ghosh said: "It makes sense that a company delivering integrated digital marketing services to global clients be fully integrated. Over the past year, iCrossing has grown to become one of the largest digital marketing companies in the world, and this integration makes us a truly global digital marketing company with search at the core."
ICrossing's clients include recognized brands such as The Coca-Cola Company, Epson, Toyota and Travelocity.
Last month iCrossing completed the integration of two firms it acquired last year, marketing technology provider Sharp Analytics and web development firm Proxicom.