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.
On an international basis Unicef has worked with agencies like TBWAChiatDay 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."
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.