"Today I lost my brother Ryan Dunn," he wrote upon hearing that Dunn had died today in a fiery car crash in Pennsylvania. "My heart goes out to his family and his beloved Angie. RIP Ryan, I love you buddy."
But Knoxville wasn't the only Jackass costar to use the social media site as an outlet to mourn.
While there's been no word yet from costar Bam Margera, his mother described Dunn as being "like a son" to her when discussing the accident with a local Philadelphia station this morning.
Earlier today, MTV released the following statement: "We're deeply saddened by the passing of a member of the MTV family, Ryan Dunn. Our hearts and thoughts are with his friends and family."