The actress attended a meet and great at the annual event and was shocked when two men became engaged in front of her.
The heartfelt moment was captured in a series of photos on Instagram, in which the pair can be seen embracing while the 30-year-old looks on in awe. Following the happy moment, John Chambrone, who presented an engagement ring to his boyfriend, Richard Owen, posted an open letter to the Oscar winner on Facebook, revealing why he chose that moment to pop the question.
"Ever since I was a kid, Carol Danvers has been my favorite character in the comics," he wrote of the character Larson portrays in the Marvel film. "Our first real date was to see 'Captain Marvel' - it was actually his (Owen) first MCU (Marvel Cinematic Universe) film and he instantly became a fan!"
"The movie meant a lot to us and I was so glad that he now shares my passion for the greatest superhero in all the known universes. Thank you again for your incredible performance of my favorite hero. Meeting you today, and being able to profess my love and commitment to my boyfriend in front of you truly meant the world to me, and to us!"
The happy couple's engagement wasn't the only heartfelt moment at the convention - Brie's fellow Marvel stars Chris Hemsworth and Tessa Thompson proudly posed alongside fans with gay pride flags. Thompson portrays openly-gay "Thor" character Valkyrie on the big screen.