Leonardo DiCaprio Wins Oscar
DiCaprio wins first Oscar after his sixth nomination
The 41-year-old had previously been nominated five times for acting's highest honour, dating back to his 1993 role in What's Eating Gilbert Grape.
But the much-coveted award eluded him until Sunday night's 88th Academy Awards, held at Los Angeles' Dolby Theatre.
DiCaprio played fur trapper Hugh Glass in The Revenant, which saw the actor plunging into icy waters to portray his character's efforts to survive.
The film's director, Alejandro Inarritu, took home the award for best director, although The Revenant missed out on the gong for best picture, which went to Spotlight, a recreation of the Boston Globe's probe into sex abuse in the Catholic Church.
Director Alejandro Inarritu and actor Leonardo DiCaprio hold their Oscars after the end of the awards ceremony at the 88th Academy Awards in Hollywood
Inarritu won back-to-back directing awards after triumphing last year with Birdman.
DiCaprio, who received a standing ovation, used his acceptance speech to drive home a message about climate change.
"Let us not take this planet for granted. I do not take tonight for granted," he said.
DiCaprio added: "Our production needed to move to the southern tip of this planet to find snow. Climate change is real, it is happening now.
"It is the most urgent threat facing our entire species and we need to work collectively together, and we need to support leaders around the world who do not speak for the big polluters and the big corporations but who speak for all of humanity."
The Revenant had gone into Sunday's ceremony with a leading 12 nominations, and was among four movies believed to have the best chances for best picture after it won Golden Globe and BAFTA trophies.
The Revenant also won Best Cinematography, but Spotlight took the night's top honour in a surprise ending to Hollywood's glittering awards season.