The agency won the business after a competitive pitch and will develop a digital strategy before rolling out creative work for the brand.
Agency Republic's role will include overseeing a major campaign for Activia that will launch in the New Year, followed by subsequent campaigns throughout 2011.
Agency Republic will work closely with Activia's marketing manager, Corinne Chant, on the digital campaigns.
The appointment does not affect Rainey Kelly Campbell Roalfe/Y&R, which will continue to handle the above-the-line creative.
While Danone has not previously been a big spender on digital, its "roller-skating babies" ad for its Evian brand, created by BETC Euro RSCG in July 2009, became one of the YouTube hits of last year, with more than 45 million views in the six months after it launched.