Eddie Redmayne presented the star with the 2016 Academy Award for Best Actress.
"Oh, wow! Thank you first to the Academy. I want to start big because the thing I love about movie making is how many people it takes to make it," Larson said before thanking the crew. "Jacob Tremblay, my partner through this in every way possible – my real partner, Alex Greenwald, I love you."
The award marked the end of what has been a whirlwind awards season for Larson, who vowed to finally read Finnegans Wake after Hollywood's biggest night.
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In a highly competitive year, Larson beat out Cate Blanchett (Carol), Jennifer Lawrence (Joy), Charlotte Rampling (45 Years) and Saoirse Ronan (Brooklyn).
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Last year, Julianne Moore won Best Actress for her performance in Still Alice.
The 88th annual Awards, hosted by Chris Rock, are being presented live on ABC from the Dolby Theatre in Hollywood, California.