The launch comes as Coke battles waning consumer interest in fizzy drinks and increasing pressure from the health lobby.
The no-calorie Zero is said to taste almost the same as the original and is billed as the most significant shake-up to its brand since the launch of Diet Coke in 1984.
It will be aimed at young men who have shied away from drinks such as Diet Coke, as it takes on Pepsi's macho Max brand.
The appointment of Lexis - which is yet to be briefed on activity for the original Coke brand - comes two months after Pepsi itself received fresh agency backing: Hill & Knowlton replaced Ketchum on the business (PRWeek, 10 March).
Lexis has worked on Diet Coke for three and a half years, and also already works on Coca-Cola's Sprite.
Joint consumer head Marilyn Wicks leads the agency's work on the brief, won after a competitive pitch. Her team reports to Coke GB head of brand PR Joan O'Connor. Fees are said to be at least £100,000.
M&C Saatchi Sport & Entertainment continues to promote Coke's football links, while Exposure still handles lifestyle PR for its 'original glass bottle'.