Damon sported an arm sling when he appeared on Tuesday's edition of the late-night chat show.
Asked what happened to him, Damon told Leno he was injured after riding down a hill and hitting a cluster of rocks hidden by dried leaves.
"I just hit it and literally the bike stopped, and I didn't. I just kept going," he said, admitting he was channeling his inner daredevil while riding with a friend.
"Yeah, like, 'Watch this move and these turns.' And, 'Watch me hit this rock!'" he laughed.
Damon said the accident happened several weeks ago, but he didn't want to wear the sling with his tuxedo at last Sunday night's Golden Globe Awards.