The partnership was formally announced at a function at HSBC's global headquarters in Canary Wharf, London.
Dyfrig John, Chief Executive of HSBC in the UK, said: "This is a fantastic opportunity for HSBC to be part of a great sporting event and to get involved in a range of programmes to support the growth of rugby at the grassroots level, both here and in South Africa."
David Hodgkinson, Group Managing Director and Chief Operating Officer, HSBC Holdings plc, said: "The British & Irish Lions is the most prestigious name in a sport that has enormous relevance to our business, clients and our people. We look forward to working with the British & Irish Lions Committee, as well as to some fantastic rugby."
The 2009 tour is shaping up to be one of the most exciting and entertaining in the Lions' 100 year history. It will certainly be among the most evocative, with the Lions facing the reigning world champion Springboks on home soil, just as in 1997 when the Lions defied their underdog status and came away 2-1 series winners.
John Feehan of the British & Irish Lions said: "We are delighted to welcome HSBC as the Lions shirt sponsor and are looking forward to an exciting and fruitful partnership over the next two years and a successful tour in South Africa."
Four 'Lions Legends' - Martin Johnson, Gavin Hastings, Gareth Edwards and Paul Wallace - attended the announcement function.
Martin Johnson, who captained the Lions in two test series including the victorious visit to South Africa in 1997, said: "The history of the British & Irish Lions is one of the strongest and most respected in sport. For the next Lions team to be travelling to South Africa to face the current world champions will make the tour one of the greatest and most exciting of the modern era."
Gareth Edwards, who played in three Lions tours between 1968-1974, said: "The values of respect, pride and good sportsmanship are what separates rugby union from many other sports. I am delighted that HSBC recognise that and want to support the British & Irish Lions."
Gavin Hastings, who played in two Lions tours and captained the team in 1993, said: "The Lions tour is a special occasion in the world of sport. Touring as a Lion was one of the highlights of my career. With the recent global explosion of interest in the sport, the 2009 tour will be the best yet."
Paul Wallace, who played in all three Lions tests on the last tour to South Africa in 1997, said: "The 1997 Tour to South Africa was a definitive moment in my career - the series was a unique victory against the odds, and with the Springboks recent Rugby World Cup triumph, the 2009 tour is sure to be just as exhilarating. It is a wonderful test for any player to be involved with and the build up is certain to start here."