Coast Digital won a three-way pitch for the business and is due to run the campaign from October to December.
It will employ paid search, affiliate marketing, email marketing and search engine optimisation to drive traffic to the Cafod World Gifts website.
Linda McBain, online marketing officer at Cafod, said: "The generosity of our supporters ensures that we can continue our vital, long-term development work in some of the world's poorest communities.
"The Christmas period is always a very important time for our fundraising activity, so it's vital that we have an effective online marketing strategy in place to help drive potential donors to their website."
James Frost, managing director of Coast Digital, said: "Cafod needed every channel to be fully accountable and produce strong ROI, so we decided on proven media channels for the digital campaign."