UN officials at a ceremony formalizing the appointment said Wonder Woman is known for her commitment to justice, peace and equality.
"Wonder Woman is an icon," said UN Under-Secretary-General Cristina Gallach. "We are very pleased that this character will help us reach new audiences with essential messages of our empowerment and equality."
UN officials have said they hoped the selection of a comic-book hero and movie character would appeal to younger women in its campaign for women's empowerment, and the slogan of the campaign says: "Think of all the wonders we can do."
Wonder Woman, a DC Comics Inc. heroine, first appeared in 1942, fighting villains, rescuing victims and unearthing diabolical plots.
On hand was actress Lynda Carter, who played Wonder Woman in a television series in the 1970s, and actress Gal Gadot, who will play Wonder Woman in a movie to be released next year.
"I couldn't think of a more perfect role for her," said Carter, saying the character shows women can be "smart and beautiful and strong and wise and kind and brave."
But the selection has been met with opposition, and nearly 1,000 people signed an online petition asking UN Secretary-General Ban Ki-moon to reconsider the choice.
"Although the original creators may have intended Wonder Woman to represent a strong and independent 'warrior' woman with a feminist message, the reality is that the character's current iteration is that of a large breasted, white woman of impossible proportions, scantily clad in a shimmery, thigh-baring body suit," the petition reads.
Several dozen UN employees, one holding a sign saying "Women deserve a real ambassador," staged a silent protest in the UN's visitor's lobby as the ceremony was held.