The agency will be responsible for spearheading the charity's digital strategy to change the way it communicates and engages with supporters and donors both on- and off-line.
Amnesty invited agencies to pitch for the business in October last year. The charity has traditionally focused on advertising to supporters through DRTV and direct marketing but now aims to target more donors online.
The first phase of activity for the agency will be to improve the use of online channels Amnesty uses to raise funds and maximise campaigns. Razorfish will initially overhaul Amnesty's website, which will relaunch in the autumn.
Earlier this month, the charity launched a campaign to appeal for information after the recent arrest and apparent disappearance of two Iranian music journalists and one of their mothers.