Dentsu has reduced its expectations for operating profit for the year to March 31 2009 from Y56.3bn (£373m) to Y46.3bn.
It also reported a 17.5% drop in operating profit for the July to September quarter to Y13.1bn.
The company is expanding its overseas presence with the acquisition of six-year old New York and Chicago full service agency McGarry Bowen for an undisclosed sum.
Tim Andree, president and chief executive of Dentsu Holdings USA, said: "We are thrilled to be able to offer Dentsu's vast global resources to McGarry Bowen, its management team and its formidable roster of clients."
The agency's clients include Kraft, Disney and HP and News Corp.
Dentsu's overseas interests include London ad agency CDP (which will be rebranded Dentsu in the new year) and US and UK-based design agency Attik.