Microsoft founder Bill Gates is donating $750m through his charitable foundation to the Global Fund to Fight Aids, Tuberculosis and Malaria.
Gates used the World Economic Forum in Davos, Switzerland, and the 10th anniversary of the Global Fund to make his announcement.
The Bill & Melinda Gates Foundation had already contributed $650m to the Global Fund, which is one of the world's largest funders of treatment and prevention for three of the developing world's most virulent diseases.
"These are tough economic times, but that is no excuse for cutting aid to the world's poorest," said Gates. "The Global Fund is one of the most effective ways we invest our money every year."
Gates gave the funds through a 'promissory note', which is a mechanism that gives the Global Fund greater flexibility to distribute it more quickly around the world.
"By supporting the Global Fund, we can help to change the fortunes of the poorest countries in the world," said Gates. "I can't think of more important work."
Since 2002 the Global Fund says it has financed 1000 programs in 150 countries, saving an estimated 100,000 lives every month.
In his fourth 'annual letter', released yesterday, Gates said that the amount spent on development was still relatively small but had made an impact on billions of people.
"The relatively small amount of money invested in development has changed the future prospects of billions of people," Gates wrote. "And it can do the same for billions more if we make the choice to continue investing in innovation."