The appointment comes in the wake of the brand's acquisition from Nestle by Premier Foods, an existing CHI client.
The agency has been charged with developing a strategy for the brand, as well as its communications, for a campaign that will break next year.
Details of its media strategy and budget have yet to be finalised.
Media for the brand is handled by Universal McCann.
Advertising for Rowntree's Jelly has lain dormant for several years, with its most famous executions developed by Ogilvy & Mather several years ago. Clive Yaxley and Jerry Gallaher developed a TV campaign for Rowntree's using themes including "Tom & Jelly" and "Botijelly".
Johnny Hornby, a partner at CHI, said: "We're excited to be handed the brief to wake up what is potentially a very exciting brand, especially one with such a strong advertising heritage."
Premier Foods was one of CHI's first clients. It won the business at the end of last year and to date has produced two TV executions for its Typhoo brand.
The Premier roster also includes Delaney Lund Knox Warren & Partners and McCann-Erickson.