The account moves from Walsh Trott Chick Smith, whose two-year tenure of the creative account ended with the agency's decision to resign it.
Ryvita spends £2.1 million on advertising, according to Nielsen Media Research, although that is expected to rise significantly to support the product launch on TV. The media assignment remains with ZenithOptimedia.
The Ryvita brand, which is owned by Associated British Foods, has had a series of abortive agency relationships in recent years.
The brand is best remembered for the "inch war" advertising created by Saatchi & Saatchi during the 70s. Since then, Lansdown-Euro, Tilby & Leeves, CDP, Abbott Mead Vickers BBDO and Publicis have worked for the brand.
Tim Mortimer, the agency's managing partner, said: "I'm aware of Ryvita's agency history. However, it's clear the brand doesn't want to keep changing agencies. It just wants to find the right one."