Hertz is centralising its pan-European marketing and Iris will primarily replace Six and Co, which will retain some areas of Hertz Europe's digital business.
It is understood that Iris will be asked to extend the brand's "Love the road" campaign work.
Hertz Europe called a pitch last month with the intent of appointing a single agency to handle all of its print, press, DM and digital advertising work.
It is believed that several above- and below-the-line agencies were involved, before Hertz shortlisted four agencies.
Elizabeth Norris, head of brand communications at Hertz Europe, said: "Our new organisation and our new ambitions for the brand have created the need for a more strategic and integrated approach across Europe.
"We felt Iris Worldwide was the right agency partner to help us deliver great creative that supports our heritage and expertise in the rent-a-car market, while building upon our 'Love the road' brand positioning."