Julianne Moore presented the The Revenant star and five-time nominee with the statuette for Best Actor on Sunday night.
DiCaprio, 41, was met with a standing ovation from his peers, including longtime friend and two-time costar Kate Winslet.
"The Revenant was a product of the tireless efforts of the incredible cast and crew I got to work alongside," he said, thanking costar and "brother" Tom Hardy: "Tom, your fierce talent on the screen can only be surpassed by your friendship off screen."
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"Making The Revenant was about mans relationship with the natural world, a world that we collectively felt in 2015 as the hottest year in recorded history," said DiCaprio, an ardent activist. "Climate change is real, it is happening right now. It is the most urgent threat facing our entire species, and we need to work collectively together and stop procrastinating ... for the billions and billions of underprivileged people who are most affected by this, for our children's children and for those people out there who's voices have be drowned out by the politics of greed."
"Let us not take this planet for granted," he concluded. "I do not take tonight for granted, thank you so very much."
Winslet was in tears watching him from the audience.
In the press room after the ceremony, DiCaprio told reporters he had been dreaming of this moment since he was 4.
"Look, I grew up in east Los Angeles. I was very close to the Hollywood studio system," he said. "But I felt detached from it my whole life, and to have had parents that have allowed me to be a part of this industry, to take me on auditions every day after school and to tell stories like this has been my dream ever since I was 4 years old, and this film to me was exemplary in the sense that I got to work with the director and all the things we spoke about off-camera during the making of this movie transferred their way onscreen."
"This was true storytelling," he continued. "We really got to have a collaborative experience together and this was a journey that I’ll never forget with Alejandro. It took up such a large portion of our lives, but as a result we have a great film to look back on for years to come."
DiCaprio's mother Irmelin Indenbirken told PEOPLE: "I'm so happy and I'm so proud of him. And even more I'm so proud of his speech, that he spoke about something so personal to him and that with his platform, the whole world will listen."
DiCaprio triumphed over Bryan Cranston (Trumbo), Matt Damon (The Martian), Michael Fassbender (Steve Jobs), and Eddie Redmayne (The Danish Girl).
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Redmayne took home this award last year for his performance as Stephen Hawking in The Theory of Everything.
Cheers erupted at various L.A. viewing parties packed with celebrities, including at Elton John's viewing party and the Kim Crawford-wine sponsored Fox Oscar party.
The 88th annual Awards, hosted by Chris Rock, are being presented live on ABC from the Dolby Theater in Hollywood, California.
With Reporting by Mike Miller and Aili Nahas