The Harry Potter star, who will be working for gender equality and the empowerment of women, tweeted her delight at the appointment.
UN Women announced the new role for the actress but shortly afterwards claimed its website had crashed such was the interest in the news.
The 24-year-old tweeted: "So excited to start work with the AMAZING people at UN Women!"
Watson, who starred in The Bling Ring and Noah, made her name in the Harry Potter movies as Hermione Granger.
In a statement Watson said she was humbled by the role which, she added, will give her the chance to "make a real difference".
"Women's rights are something so inextricably linked with who I am, so deeply personal and rooted in my life that I can't imagine an opportunity more exciting," she said.
"I still have so much to learn, but as I progress I hope to bring more of my individual knowledge, experience and awareness to this role."
Phumzile Mlambo-Ngcuka, Under-Secretary-General and Executive Director, UN Women, paid tribute to Ivy League graduate Watson's "intellect and passion".
"We are thrilled and honoured to work with Emma, whom we believe embodies the values of UN Women," said Ms Mlambo-Ngcuka.
"The engagement of young people is critical for the advancement of gender equality in the 21st century, and I am convinced that Emma's intellect and passion will enable UN Women's messages to reach the hearts and minds of young people globally."
Watson, who graduated from Brown University in America earlier this year, has already been involved in the promotion of girls' education, visiting Bangladesh and Zambia as part of her humanitarian efforts.
In the first appointment under Ms Mlambo-Ngcuka's leadership, Watson will serve as an advocate for UN Women's HeForShe campaign which calls for men and boys to stand up against inequalities faced by women and girls around the world.