Now was hired following a pitch through AAR and will work on campaigns across all media.
The agency will assist WaterAid as it moves towards a target of universal clean water access by 2030.
Marcus Missen, the director of communications and fundraising for WaterAid, said: "We have chosen Now to be our partner to develop our UK communications as part of our continued growth."
Mark Lund, the managing partner of Now, said: "We can’t imagine a better cause or organisation to be working with."
Last year, WaterAid launched its Big Deal Appeal, which won support from media owners including UTV Media and GMG Radio.
Presenters on radio stations encouraged listeners to sponsor them and donate to WaterAid, in a bid to raise £1.2 million towards providing clean water and sanitation to more than 134,000 people in Malawi.