In his State of the Union address, Mr. Bush asked Congress to budget $10 billion in new money and $5 billion in already allocated assistance over five years to provide AIDS drugs to 2 million Africans, help prevent 7 million new infections, and care for those infected with the virus and children orphaned by the disease.
He called the money a "work of mercy" that would save millions of Africans.
"This nation can lead the world in sparing innocent people from a plague of nature," Mr. Bush said in his address Tuesday.
Stephen Lewis, the United Nations special envoy for HIV/AIDS in Africa, praised the announcement as a "dramatic signal" the United States is ready to confront the pandemic.
"It gives leverage to activists everywhere to keep the pressure on. It transforms the response. It opens the floodgates of hope," he told a news conference in Johannesburg.
Mr. Bush's pledge should also put pressure on other developed countries to increase their contributions to AIDS, he said.
Experts estimate rich nations need to spend at least $10 billion a year to fight HIV in the developing world, but spent only $2.8 billion last year.
Despite his optimism, Lewis questioned what programs the already allocated $5 billion would come from and what share of the money would go to the United Nations' global fund to fight HIV.
An estimated 29.4 million people in sub-Saharan Africa are infected with HIV, out of 42 million worldwide infections.
The virus has orphaned 11 million African children, decimated work forces across the world's poorest continent and slashed life expectancy and overwhelmed health care systems.
Only several thousand have access to advanced AIDS medicine commonly available in developed countries.
Some African officials said they needed more details of Bush's offer.
"We appreciate the move and the gesture, but will be it enough? We don't even know how the money will be used and who will get what," said Najib Balala, Kenya's social services minister. "How do we all share this money."
"We suffer in Africa — there's a lack of awareness, a lack of drugs and hospital facilities needed to treat AIDS ... these are the things that need to be put in the place," he said. "The biggest gift would be free drugs or subsidized drugs and $3 billion a year is very little money
Prega Ramsamy, executive secretary of the Southern Africa Development Community, said the money would need to go small-scale community projects to have any real effect on people's lives.
"We need to make sure that it filters down to the level of people that most need it," he said.
Maimouna Dieng, national program coordinator of the Senegalese Association for the Well-being of the Family, called the pledge of money a "godsend" and hoped it would provide "an opportunity to put our programs into practice."