The actress was given the title on Friday (November 14), in recognition of the work she’s done to bring violence against women to an end.
The United Nations’ Secretary-General, Ban Ki-moon, explained that the star has used her voice, her compassion, and her special relationship with the public to make the world a better place.
Theron joins nine other messengers of peace, including George Clooney, Michael Douglas and musician Yo-Yo Ma.