As a partner at the group's consumer insight specialist, Zalpha, she will oversee brand planning for the group's four UK agencies.
As well as working on the branding aspect of client projects for Zalpha, Rossi will perform a similar role across the client base of the WWAV offices in London, Bristol, Edinburgh and Leeds.
As Grey's chief planning officer, Rossi was one of the management team at the agency before the arrival of Garry Lace as its chief executive last October. She was a member of the Grey team that pitched for the Electrolux global account recently.
Rossi, whose agency career includes spells at Saatchi & Saatchi, TBWA/GGT and Allen Brady & Marsh, has been at Grey for six years.
She will leave at the end of the month before taking up her new role in March.
"I've always been fascinated by the idea of being able to see the effect of my work," she said. "In direct marketing, you are much more able to evaluate what works and what doesn't. I have been working in above-the-line agencies for more than 20 years now and I feel the need to get a much broader communications perspective."
Rossi currently serves as a member of the Account Planning Group with responsibility for training, and part of her new role will be to nurture and develop the existing brand planning team at WWAV.