"I will always defend what right but I was wrong to put down Mark Rylance and I apologize. He is a fine actor but I thought Sly deserved it," he tweeted Monday.
I will always defend what right but I was wrong to put down Mark Rylance and I apologize. He is a fine actor but I thought Sly deserved it— Frank Stallone (@Stallone) February 29, 2016
The Creed star lost the award to the Bridge of Spies actor, and while Sly Stallone appeared gracious in defeat, his little brother vented his frustrations to Twitter.
"The Academy should be ashamed of themselves," the actor-musician wrote earlier Monday. "It's as clear as the nose on your face that Sly won."
Frank isn't the only person surprised by the results. Coming into the Oscars, Stallone was a favorite to win for his latest turn as Rocky Balboa after winning awards at the Golden Globes and Critics Choice Awards.
Instead, the prize went to fellow frontrunner and three-time Tony winner Rylance, who was also considered a favorite for his role as Soviet super-spy Rudolf Abel in Steven Spielberg's Cold War drama.
Although Stallone was a fan favorite, his brother's initially harsh reaction garnered criticism on social media.
"Maybe, like Colbert, 'Frank Stallone' is a 'character' Frank Stallone plays. Though he IS the best Frank Stallone we've got," one commenter remarked.
Another, meanwhile, pointed out, "Mark Rylance is one of the best stage actors of all time and one of the best in general. Sad for Sly but no need to be insulting."
The Creed actor later took to Instagram to send a message to his supporters, writing: "To all the 'real Rocky's' of the world, Please hang on to your dreams."
Stallone has received an outpouring of support from fans and celebrities alike.
Arnold Schwarzenegger took to Instagram to offer a consolation video, captioning, "To me, you're the best, no matter what they say."