The "King Kong" actress just returned from a trip to Zambia, where she saw hospitals, homes and schools where lives have been destroyed by AIDS.
"Given these stark realities, I can no longer stand on the sidelines," Watts told a news conference Monday. "I want to use this position as a way to spread the word, to tell people what I saw."
Her appointment came as UNAIDS unveiled a new campaign called, "4 for Everyone," which seeks to give everyone the right to HIV/AIDS prevention, treatment, care and support.
The program estimates that 40 million people now live with HIV, the virus that causes AIDS, and that 25 million people have died from AIDS.
"I have seen that every one of us can make a difference — and I am anxious to join UNAIDS in this effort," Watts, 37, said.