Bachchan's appointment as a 46664 Special Ambassador was to be announced at the International Film Festival of India in Panaji, Goa, South African film company Videovision Entertainment said.
The 46664 campaign - named after Mandela's prison number - aims to raise awareness of the global HIV/Aids pandemic, as well as funds for the Nelson Mandela Foundation which supports programmes for the prevention, testing, care and support of those infected with the virus.
Other 46664 special ambassadors include US former president Bill Clinton, Hollywood actors Brad Pitt and Will Smith and talk show host Oprah Winfrey.
The appointment of Bachchan was to be announced by the chief executive of the Nelson Mandela Foundation, John Samuel, after a screening of the film Yesterday at the festival on Wednesday.
Bachchan is widely known for his activism against HIV/Aids and will be endorsing anti-Aids campaigns in both India and South Africa, which have the highest infection rates of the disease in the world, the statement said.
Yesterday, produced by well-known South African filmmaker Anant Singh and directed by Darrell James Roodt, follows the plight of a rural woman in South Africa - played by South African actress Leleti Khumalo - who falls ill and discovers she is HIV-positive.
Mandela's prison number is used
to create awareness of Aids
The movie is the first to be supported by the Nelson Mandela Foundation and is South Africa's official entry for the 2005 Academy Awards in the Best Foreign Language Picture category.
It is to be the highlight of Wednesday's screenings at the festival, which has as its focus World Aids Day.