The news will come as a huge relief for Octagon after a high-profile financial slump within its motorsports division.
Dentsu, however, will sell TV rights to Japan and Asia.
Octagon, which has proved its success at selling soccer to a global audience - it handles the English Premier League's overseas sales - beat off competition from TEAM Marketing, Sportfive, IMG and KirchSport.
It will be a big blow to IMG which lost out earlier this year when UEFA awarded long-time partner TEAM the contract to handle agency rights for its Champions League tournament.
