The appointment came a day before the start of the annual debate at the UN General Assembly. Aishwarya has been involved in humanitarian issues for many years and will now have a special focus on HIV/AIDS, Xinhua reported.
At a press conference Monday, UNAIDS Executive Director Michel Sidibe welcomed Aishwarya to the UNAIDS family.
"Mrs. Rai Bachchan is respected and admired by millions of people around the world," Sidibe said. "I am convinced that through her global outreach, Mrs. Rai Bachchan can help UNAIDS reach its goal of eliminating new HIV infections among children by 2015."
Speaking on this occasion, Aishwarya said: "I am honoured to accept this appointment."
"Spreading awareness on health issues, especially related to women and children, has always been a priority for me," she said.
"And now, as a new mother, I can personally relate to this -- the joys and concerns of every mother and the hopes that we have for our children."
"I strongly believe that every baby should be born free from HIV. And I wish that every woman living with HIV stays healthy and has access to treatment. I promise that with UNAIDS, I will do my utmost to make this happen," she added.
Aishwarya worked as a model before starting her acting career, and won the Miss World pageant in 1994. She has starred in films in various languages including Hindi, English, Tamil, Telugu and Bengali.