UNAids said 22 countries in the world's worst affected region had seen a drop in new cases of more than 25%.
The fall was because of greater awareness and better use of preventative measures, it said.
But UNAids also noted that cases of HIV were increasing in Eastern Europe and Central Asia, and among gay men in developed countries.
Michel Sidibe, UNAids executive director, said the world was making "real progress" towards achieving the sixth Millennium Development Goal (MDG6) of halting and reversing the spread of HIV/Aids by 2015.
"For the first time change is happening at the heart of the epidemic. In places where HIV was stealing away dreams, we now have hope," he said.
UNAids says there are now 5.2 million people worldwide receiving treatment for HIV/Aids, which has helped to ensure that 200,000 fewer people died from the virus in 2008 than in 2004.
The agency said young people "are leading the prevention revolution by choosing to have sex later, having fewer multiple partners and using condoms, resulting in significantly fewer new HIV infections in many countries highly affected by Aids".
The use of male condoms has also doubled in the past five years, while the report notes that "tradition is giving space to pragmatism" in many communities as they embrace male circumcision, which research shows has the potential to reduce HIV infections among men by nearly 60%.
'Challenges remain'
China, where cases are largely concentrated within high-risk groups, was praised for its efforts to increase preventative measures for drug users.
UNAids said South Africa had also rapidly increased "efforts to achieve universal access to HIV prevention, treatment, care and support".
But there was a warning that "challenges remain" in the global fight against HIV/Aids, including expanding epidemics in Eastern Europe and Central Asia, and resurgence in new infections in wealthier nations among men who have sex with men.
The UN also called for greater investment in HIV/Aids prevention, warning that there was a $10bn (£6.4bn) shortfall in 2009.
It said those countries most severely affected by HIV/Aids could not handle the crisis with their own resources alone.
"At this turning point flat-lining or reductions in investments will set-back the Aids response and threaten the world's ability to reach MDG 6," said Mr Sidibe.
"Investing for Aids is a shared responsibility - between development partners and national governments."
BBC News
