Rossi, who left Grey at the end of last year where she was planning chief and chair of planning best practice for Grey Europe, will head brand planning across Zalpha's four UK offices.
The move sees her take the unusual step of crossing over from an above-the-line agency to below the line.
Rossi said: "I am very excited to be joining an organisation that puts brand understanding, consumer and data insight and account planning at the very heart of its philosophy."
Her departure from Grey came on the heels of the news that the former head of TBWA\London Garry Lace had been confirmed as the new chief executive of Grey.
However, although it was reported that her exit from the agency was a result of Lace's appointment, she denied this.
At the time, she told 北京赛车pk10 magazine: "My decision to leave has nothing to do with Garry's arrival. The fact is that I've been in the planning departments of large agencies for more than 20 years and it's time to do something different."
Zalpha was spun out of WWAV three years ago and now boasts a team of 40. It produces branding, e-commerce, direct marketing and database strategies that are then put into action to deliver measurable and accountable results.
The company has offices in London, Edinburgh, Leeds and Bristol.
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