The agency is understood to be in contract discussions with Diageo, Guinness' parent company. It picked up the brief after a head-to-head shoot-out against Guinness' other global roster agency, Abbott Mead Vickers BBDO.
Saatchis' London office was the lead shop on the pitch, which started in November last year.
The agencies were briefed to create an enormous media spectacle, and surrounding campaign, that will be the showcase for the celebrations.
The strategy to promote the anniversary will be the main driver for most of the company's communications spend next year.
It is not thought that the appointment will affect the company's advertising relationships around the globe.
BBDO holds the Guinness account in the UK, Ireland and North America, while Saatchis has the business in Asia, Africa and South America.
Guinness' most recent UK campaign was the Abbott Mead Vickers BBDO-created "tipping point", which consisted of an online treasure hunt and a 60-second TV ad. Guinness ploughed £10 million into the work.
Diageo denied an appointment had been made.