Although no figures have been revealed by tournament organisers, it is understood in its first year to be worth less than the amount paid by 2018 sponsors NatWest for a one-year deal.
It is thought, though, that Guinness’ backing will increase year on year during the term of its support for European rugby union’s blue riband event.
Guinness currently sponsor the PRO14 competition, while they are also previous title sponsors of the English Premiership.
The deal will begin for the 2019 tournament, which kicks off on February 1 in Paris when France host Wales.
Six Nations chief executive Benjamin Morel said: “We are absolutely delighted to enhance our long-standing relationship with Guinness, and they will be a terrific title sponsor for rugby’s greatest championship.
“We could not be more excited to be working with the Guinness team over the coming years on developing new and innovative ways to connect with our fans and consumers.”
Diageo own the Guinness brand, and their president for Europe, Turkey and India, John Kennedy, added: “We are really looking forward to the start of the Guinness Six Nations in February.
“While working with the Six Nations, we want to enhance both the Championship and the fan experience.”
Ireland, recent conquerors of world champions New Zealand and Six Nations Grand Slam champions last season, will start the 2019 tournament as firm favourites.