Grey New York was given a second place by the jury while Ogilvy Brazil, Sao Paulo, was placed third.
The gong is awarded to the agency that received the most number of points during the festival last week.
The Interpublic Group shop picked up awards for its work with the Ad Council for "love has no labels" (in the video, below), Hammerhead for "Hammerhead navigation" and Equinox for "the pursuit".
Adam & Eve/DDB took home the trophy for the UK last year, the only British winner of the award since 2002 when it was won by Saatchi & Saatchi.
The UK has produced six Agency of the Year winners since the award was launched in 1993. Bartle Bogle Hegarty won the inaugural and again in 1994, followed by Lowe Howard Spink in 1995 and Saatchi & Saatchi in 1996.
Watch: Claire Beale, the global editor-in-chief of Campaign, interviewed Bob Greenberg, founder and chief executive of R/GA, at Cannes last week.