Schwarzenegger, 68, is showing his support to his support to his close friend after the actor lost the Oscar for Best Supporting Actor to Mark Rylance on Sunday night.
In an Instagram video, Schwarzenegger tells Stallone he's proud of him.
"Sly, no matter what they say, remember, to me you are the best," he said in the clip. "You were the winner, I'm proud of you."
After Rylance was announced as the winner, the crowd was a mix of cheers and stunned silence.
Check out PEOPLE's full Academy Awards 2016 coverage and complete winners list!
And Schwarzenegger wasn't the only one to speak out about Stallone's loss. Fans swiftly took to social media to express their shock and show their support for Stallone.
"You're my hero," wrote one user.
"So pissed right now that @TheSlyStallone did not win for Creed. An absolute travesty!" wrote another.
For all things Oscars, click here to head over to PEOPLE's Snapchat Discover, where we're breaking down the best dresses of the nights, most meme-orable moments and everything in between.
Mark Rylance is wonderful!! I just assumed Sylvester Stallone would win— mia farrow (@MiaFarrow) February 29, 2016
The 88th annual Awards, hosted by Chris Rock, are being presented live on ABC from the Dolby Theatre in Hollywood, California.