Dallyn joins R/GA following a three-year spell as art director at website development agency Blast Radius. He will report to R/GA London's executive creative director James Temple and design director Lucio Rufo.
The new art director will lead the London office's Nike account, liaising with R/GA's New York-based Nike team. Dallyn has previously worked with brands such as AOL, Nintendo, Nokia and Orange.
Temple said: "Through his incredible portfolio, Sam shows that he has the ability to distil challenging client directives to create visually stunning websites, subtle yet powerful brand identities, and striking print campaigns."