The network scooped the gong at the awards ceremony in Brussels last night, after taking four trophies in three categories: FMCG, pharmaceutical and healthcare, and automotive.
Grey Copenhagen took a gold award for the holistic communications campaign "As it should be" for Coke Zero, while Grey London took a silver for P&G Lenor and its "Disappear the sustainability initiative" campaign.
Grey London also won a bronze for GSK Corega and its "Celebrity Testimonials" campaign, and Atletico International won a bronze for Seat Altea XL and "Toys".
Carolyn Carter, president and CEO of Grey Group Europe, Middle East & Africa, said: "In today's tough business environment, driving brand growth through effective communications is more important than ever before.
"Our success year after year is a great testament to the successful teamwork between our agencies and our clients."
The Euro Effies are organised by the European Association of Communications Agencies (EACA), and reward campaigns that build brands across two or more EU countries.
Dominic Lyle, director general of EACA. said: "Grey EMEA has demonstrated once again a unique ability to deliver proven effective business results for its clients - across categories and across communications disciplines. To win Agency Network of the Year for the fourth year in a row is nothing short of outstanding."