Neither Euro nor Sanofi-Aventis would confirm the size of the appointment, but a Euro RSCG source did reveal that it would make the pharmaceutical company the third largest global Euro client, measured on billings.
Euro RSCG Life, the network's medical and healthcare arm, will also be handling additional assignments for the range of thrombosis and diabetes drugs available from Sanofi-Aventis.
In addition, Euro RSCG C&O has won the drug company's corporate communications account, previously held by Publicis in Paris.
The Euro RSCG Worldwide chief executive, David Jones, said: "This has been a great year for the agency, and a wonderful way to end it."
Euro RSCG has handled a number of assignments for the company since its formation in 2004 after the takeover of Aventis by Sanofi-Synthelabo in a deal worth EUR52 billion.
Earlier in November, Euro RSCG Life unveiled changes to its management structure. Jim Metropoulos joined as the worldwide chief executive from WPP's healthcare division, Sudler & Hennessey.
His appointment followed the promotion of the former Euro RSCG Life chief executive, Ron Pantello, to the role of chairman.