The US company will be briefed to create interactive brand campaigns, websites, customer management programmes, mobile marketing and interactive television.
In a statement, Youcef Nabi, the client international general manager for L'Oreal, said: "R/GA has impressed us with its ability to present a 360-degree vision of the internet experience, while respecting the highly specific image and identity of the L'Oreal brand. Being a premium brand today means providing real at-home services."
The agency will work across the company's cosmetics, skincare and haircare brands, including Lancome, the Garnier range of products, Vichy, Biotherm, La Roche Posay and L'Oreal Professional.
The business was put up for pitch in December with nine networks battling it out over two presentations, one in the US and one in Paris.