The agency saw off Joshua, Large and XM. The incumbent agency was SMP.
Saint has been briefed to redesign the Volvic website for its parent company, Danone, and integrate it with the current TV campaign. Saint will also handle the online launch for Volvic Revive, a vitamin-fortified variant to be introduced to the UK later this year.
It will raise awareness of the brand via online advertising, virals and games.
Jo Tinworth, the marketing manager at Volvic, said: "Online is where we will find our consumer and is the perfect medium to fully express the personality of the brand."
The appointment is Saint's first piece of digital business since it was founded by David Gamble and Simon Labbett at the start of this month.
RKCR/Y&R already holds the creative advertising account for Danone.