The brand will become the official beer of the tour, and follows previous sponsorship deals between Guinness and the sport, including involvement with the Rugby World Cup in 1999 and ongoing work with London Irish RFC.
Guinness marketing director Nick Robinson said: "Guinness has had a long association with rugby at an international and club level and now we are delighted to be on board as the official beer for the 2005 Lions' tour."
John Feehan, Lions chief executive, said: "Guinness is a very welcome addition to our family of partners."
If you have an opinion on this or any other issue raised on Brand Republic, join the debate in the Forum here.