“It is a great honour to be chosen as a champion for people who live on the fringes of society and struggle every day to maintain their dignity and basic human rights," the Nobel Peace Prize laureate said in a statement.
"I would like to be the voice of the voiceless,” she added.
UNAIDS executive director Michel Sidibe said the agency was honoured she had accepted the task.
"From small villages to big cities, from Africa to Asia, people are talking about Daw Aung San Suu Kyi.... She is inspirational," he said in the statement.
Suu Kyi, who has been named UNAIDS Global Advocate for Zero Discrimination, will work to "eliminate stigma and discrimination", which she fears can "create an environment of fear that prevents people from accessing life-saving HIV services," the agency said.
"It is important that everyone who suspects they may be at risk seeks an HIV test and knows their HIV status early, so they can prevent new infections and can access life-saving treatment when needed,” she said.