It means the three-match campaign will be known as the Castle South Africa 2009 Lions Series - with the sponsors adding an extra R3m to their R22m backing as a bonus for winning the series.
The record-breaking agreement - the richest in Springbok rugby history - will add even greater significance to the Lions' return to South Africa for the first time since they won the last series 2-1 in 1997.
The Lions 10-match tour kicks-off in Rustenburg on Saturday, May 30 against a High Veld XV and takes in matches against four provincial teams, a second invitational side and the Emerging Springboks.
"This sponsorship not only reaffirms SAB's commitment to South African rugby and its fans, it also rubber stamps the company's support for traditional rugby tours, which have vanished over the past few years," said Ian Penhale, SAB marketing director.
"The rich tradition and camaraderie associated with this unique tour reflect the very values of SAB. True rugby fans have been yearning for the traditional rugby tours, and the British & Irish Lions tour has earned a reputation as the last of the great rugby tours.
"So we are proud for Castle Lager to be associated with this prestigious event. This is a limited edition tour and everything about it is special. We want the Castle South Africa 2009 Lions Series to be the tour that stands the test of time."
South African Rugby Board president, Oregan Hoskins, said he felt the 2009 tour by the Lions would be "the best yet".
"The British & Irish Lions tour is a like a mini Rugby World Cup to me. Its significance and uniqueness makes the tour important for SA Rugby and the nation, and we are honoured to host the team - we think this will be the best British & Irish Lions tour yet.
"We are very pleased to have such a reputable sponsor on board for this tour and excited to see Castle Lager reunited with the world of rugby once again. We have a relationship dating back several years and its brand support is important for rugby and everything it stands for."
A Springbok team sponsor in the past and, more recently, an associate sponsor, SAB were the title sponsors of the unforgettable 1997 British & Irish Lions tour.