The agency won the brief following a four-way pitch. The appointment will strengthen the agency's existing relationship with Unicef, which it works for in the UK.
WPN will be responsible for creating activity that will recruit new regular givers, retain and upgrade existing donors, and regain lapsed donors. The agency will work in territories including the UK, the Netherlands, Canada, France and Sweden.
In the UK the charity has recently worked with a number of agencies, including Abbott Mead Vickers BBDO, Rapp and Krow. On an international basis the charity has used TBWA\Chiat\Day LA and Droga 5.
Owen Watkins, Unicef International fundraising manager, said: "We are working with WPN to develop a concept that communicates the sheer scale, reach and effectiveness of Unicef in a compelling way for donors, and that works across our fundraising markets globally."
Gail Cookson, strategy partner at WPN, said: "This is an enormous opportunity to deliver not only strategy and planning, but also compelling communications that are grounded in insight. Our creative guys are inspired at the prospect of this campaign."
Separately, Unicef is seeking a global agency to handle a campaign in the run-up to Christmas. Proposals need to be submitted by 15 June as Unicef gears up for the festive fundraising activity.