As part of a joint UN initiative, the actress will raise awareness of the issues relating to the prevention of the HIV virus in children.
Speaking to PTI, the 39-year-old actress said: "I am honoured to accept this appointment.
"Spreading awareness on health issues specially related to women and children has always been a priority for me 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."
Rai Bachchan, who gave birth to her first child in November 2011, said she was committed to ensuring that the aim of the programme is achieved.
"Ït seems the perfect time because I am a new mother, and I am talking about mother to child transfer and prevention of HIV," she continued.
"This genuinely has been a cause close to my heart. What works today for me is that I am... able to plan my time at this point of time in my life because I have taken a break. I promise that with UNAIDS I will do my utmost to make this happen."
The official announcement of Rai Bachchan's appointment was made at the UN headquarters by UNAIDS Executive Director Michel Sidibe.
While her father-in-law Amitabh Bachchan has served as the International Goodwill Ambassador for UNICEF, the new role represents the first official association with the United Nations for Rai Bachchan.
She met last week with UN Secretary General Ban Ki-moon and actor Michael Douglas at a ceremony to commemorate the International Day of Peace.
UNAIDS is committed to providing universal access to HIV treatment, prevention, care and support, and the new initiative focuses on 22 countries including India, where over 90 per cent of all new HIV infections are among children.
The goal of the UN programme is to eliminate new HIV infections among all children by 2015.