Nestlé launched the review last month, and the incumbent Mindshare chose not to repitch.
Arena won the business following a competitive process that saw agencies including Initiative, Maxus and the7stars, pitch for the account.
Jenny Craig operates a weight-loss programme through one-on-one consultations or phone support at home, and has an established following in the US, Canada and Australia.
Nestlé, which hopes to establish the brand in Europe as a serious competitor to WeightWatchers, launched the brand in the UK and France 12 months ago.
The bulk of Jenny Craig customers are women in the US. The brand has made use of female celebrities, including the US actress Kirstie Alley, in its TV advertising.
In the UK, it has run advertising featuring the TV presenter Cheryl Baker.
The review was led by Nestlé's media controller and agency liaison, Ian Pawsey.