Leonardo DiCaprio has been appointed a United Nations Messenger of Peace to help promote global action on climate change.
Secretary-General Ban Ki-moon said the American actor "is not just one of the world's leading actors" but has "a long-standing commitment to environmental causes".
DiCaprio has established a foundation with a mission to promote the Earth's last wild places and build a more harmonious relationship between humanity and the natural world.
The secretary-general said the actor will address next Tuesday's climate summit, where he expects some 120 world leaders.
DiCaprio said he was honoured by the role and feels "a moral obligation to speak out at this key moment in human history" about the threat of climate change.
The 39-year-old Wolf Of Wall Street star founded the environmental foundation in 1998.
He joins 11 other prominent world figures who advocate on behalf of the UN as Messengers of Peace, including Stevie Wonder, Michael Douglas, George Clooney, Brazilian author Paulo Coelho and conductor Daniel Barenboim.