Leonardo DiCaprio arrives at the British Academy of Film and Arts (BAFTA) awards ceremony at the Royal Opera House in London February 16, 2014. [Photo/Agencies]

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DiCaprio will address a UN summit meeting on climate change on Sept 23, a day before the UN General Assembly's annual gathering of world leaders begins. Many leaders from the 193 member states of the United Nations are expected to attend the climate session.

"Leonardo DiCaprio is not just one of the world's leading actors," Ban said. "He has also a long-standing commitment to environmental causes, including through his foundation."

In 1998, the actor established the Leonardo DiCaprio Foundation, whose mission is protecting Earth's last wild places and building a more harmonious relationship between humanity and the natural world, the United Nations said in a statement. The organization has contributed funds toward protecting tigers in Nepal, elephants in the wild, and marine animals, including sharks.

"I feel a moral obligation to speak out at this key moment in human history - it is a moment for action," DiCaprio was quoted as saying in the statement. "How we respond to the climate crisis in the coming years will likely determine the fate of humanity and our planet."

DiCaprio, 39, added that it was an honor for him to accept the post.

The Academy Award-nominated star of "The Wolf of Wall Street" and "The Aviator" joins other celebrities past and present, from Audrey Hepburn to Angelina Jolie, who have represented various UN organizations.

There are currently 11 other messengers of peace and one goodwill ambassador, the United Nations said. Other messengers who advise the world body on a variety of issues include actors Michael Douglas and Charlize Theron and author and Holocaust survivor Elie Wiesel.