Agenda will move into WPP's relationship and digital marketing network Wunderman.
It offers services including integrated portals, digital marketing, eCRM and enterprise application integration.
Sorrell said: "Agenda represents an Asian digital jewel, much coveted by our competitors. Together we will further distance ourselves from our competition by focusing on adding to our Asian and digital capabilities, bang in line with our strategy."
Agenda employs 260 people and is based in Hong Kong, with offices in Beijing, Shanghai, Malaysia and Taiwan. Clients include Johnson & Johnson, Pepsi, HSBC, French supermarket chain Carrefour and DHL.
The agency's unaudited revenues for the year ended June 30 2007 were $9.2m (£4.7m), with gross assets at the same date of $6.2m.
The deal is WPP's first Asian acquisition of 2008 -- the group bought into four Chinese companies alone in 2007, including agencies MeThinks, Dawson Integrated Marketing Communications and Star Echo, as well as China Broadband Capital Partners, the venture capital outfit behind the launch of MySpace in China.