The Led Zeppelin star was on the show to promote his new album, Lullaby and... The Ceaseless Roar, when he stunned the host with a gift from his shirt pocket.
Plant said, "I've brought you a little present from our stock room."
Colbert awkwardly accepted the gift, saying, "What is that...? Let's find out what this is later, shall we? And for the purposes of my lawyer and the network, this is a cigarette."
The host then pocketed the rolled joint.