"As an actor, I pretend for a living. I play fictitious characters often solving fictitious problems," the actor told the summit of 120 leaders.
"I believe mankind has looked at climate change in that same way -- as if it were fiction."
Sporting a ponytail and a suit and tie, DiCaprio took to the podium at the invitation of UN Secretary General Ban Ki-moon to address a one-day summit billed as the largest gathering ever of world leaders on climate change.
The actor, who has been vocal on issues of climate change through the "Leonardo DiCaprio Foundation", called for setting a price on carbon emissions and eliminating government subsidies for oil, coal and gas.
"This is the most urgent of times and the most urgent of messages. Leaders of the world, I pretend for a living, but you do not," he said.
"Now it is your turn, the time to answer humankind's greatest challenge is now. We beg of you to face it with courage and honesty.