Fetch, whose headquarters are in London, will retain its own brand identity within the network and continue its aggressive expansion strategy, with a particular focus on Asia and North America.
The non-executive shareholders are realising the value of their shares immediately. The remaining shareholders, including the chief executive and co-founder, James Connelly, are believed to be working to incentivised earn-outs.
Fetch, which launched in 2009, employs 96 people and services clients (including eBay and Hotels.com) in 90 countries. The agency is reportedly on track to double its turnover this year, generating about $100 million (£62.7 million) from ad revenues.
The deal was negotiated by Connelly, Greg Grimmer, the chief operating officer of Fetch, and Rob Horler, the chief executive of Dentsu Aegis Network Northern Europe.
Connelly called the sale "a logical next step", as the agency looks to scale globally.
He added: "Dentsu Aegis Network has a proven track record of growing digital businesses and a clear appreciation of the increasing importance of mobile in the media landscape."