Harvest will develop a digital strategy encompassing affiliate, search and creative work to promote the charity's cause.
Care International's first major online campaign, created by Harvest, launched last week. It contains videos, embedded in banner ads, of impoverished children.
Care's previous online activity was limited to search marketing, but the appointment of Harvest marks investment in another channel, allied to Care's usual strategy, which encompasses TV and print.
Harvest also works on the digital accounts for charities RSPB and Shelter and has recently launched campaigns for Auto Trader, Tesco Personal Finance and Norwich Union Healthcare.