"Don't know how I am gonna play 2night but I am for AM," Barker posted on Twitter Saturday night. "My brother is gone. I love u and miss u, I'll never forget all the good times we had."
He continued: "I'll never forget everything we've been through and every time I play the drums, I'll think of you. U were an amazing friend/DJ/human being."
Both Barker and AM survived a plane crash in 2008 that killed four other people.
"Rest in peace, my brother," he Tweeted. "This really f***s me up."
This must be so hard on Barker because they were very close. We hope he stays strong!