The appointment was made without a pitch. Lowe, a Nestle Rowntree roster agency, has recently created work for other Nestle brands, including Aero and Milkybar Munchies.
Double Cream, Nestle's first new chocolate bar for five years, will be targeted at women and has just gone on sale in some shops.
Nestle Rowntree's creative account is split between Lowe and J. Walter Thompson and Roose. JWT is the lead agency on its chocolate brands and handles advertising for brands including Yorkie and Kit Kat.
Lowe is creating TV advertising for the launch, which is intended to bring the qualities of premium chocolate to the mainstream sector.
Jonathan Rigby, a board account director at Lowe, said: "Nestle Rowntree has pushed us to deliver a genuinely fresh campaign for the sector. When an agency gets a brief like that, anything less than a blockbuster launch would be an opportunity lost."
Jenny De Saulles, a Nestle Rowntree senior brand manager, said: "Our expectations for Double Cream are extremely high. Early trials are promising. We're confident that Lowe will help deliver the great launch advertising that the product deserves."