The star took home the award for Best Supporting Actor at Sunday night's Academy Awards ceremony for his heralded turn as a transgender AIDS patient in the acclaimed drama, opposite Matthew McConaughey.
In accepting his award, Leto, 42, paid tribute to his mother, Constance, who sat alongside the actor and his brother Shannon.
"In 1971," Leto said, "there was a teenage girl who was pregnant with her second child. She was a high-school dropout and a single mom, but somehow she managed to make a better life for herself and her children."
"That girl is my mother, and she's here tonight," he continued, adding: "And I just wanna say, I love you, Mom. Thank you for teaching me to dream."
Wrapping up his speech, Leto, dressed in a white tuxedo top, said: "This is for the 36 million people who have lost the battle to AIDS. And to those of you out there who have ever felt injustice because of who you are or who you love, tonight I stand here in front of the world with you and for you."
A first-time nominee, Leto famously took a six-year hiatus from making movies before signing on for Dallas Buyers Club, choosing instead to tour with his band, Thirty Seconds to Mars.
Backstage, shortly after winning his award, Leto kept the press room in stitches when he handed off his Oscar to pass around.
"Take a fondle," he cracked. "Who's your favorite Oscar winner now?"
Leto was up for Best Supporting Actor against Barkhad Abdi in Captain Phillips, Bradley Cooper in American Hustle, Michael Fassbender in 12 Years a Slave and Jonah Hill in The Wolf of Wall Street.
With reporting by MICHAEL FLEEMAN
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