The Diageo-owned Irish brewer replaces NatWest, which had a one-year agreement for 2018 following the termination of a 14-year sponsorship by parent Royal Bank of Scotland (RBS).
No financial details were given but media reports have spoken of a "cut-price" deal for a tournament that starts in February.
The Times newspaper said Guinness would pay six million pounds in the first year, compared to a reported nine million paid by NatWest, but with the sum growing annually to double by the final year.
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The annual tournament between England, Ireland, France, Wales, Italy and Scotland will be rebranded as the Guinness Six Nations.
Guinness already sponsors the PRO14, involving professional teams from Ireland, Scotland, South Africa and Wales, and has partnerships with Irish clubs Leinster and Munster as well as the four home unions.
"This announcement affirms our long-standing support for the game of rugby and cements our position as one of the world’s biggest rugby supporters," said Mark Sandys, who oversees Diageo's beer brands.
(Reporting by Alan Baldwin, editing by Toby Davis)
