Jennifer Aniston's fiancée made the revelation at the Free Arts Annual Auction in New York on Wednesday while speaking about his friend Scott Campbell, who is married to actress Lake Bell.
"We became friends first, and then we had a couple drunken tattoo nights," Justin said of tattoo artist Scott. "And I've tattooed him, too, by the way."
Justin Theroux and Jennifer Aniston
Justin went on to describe the inkings he's given his friend.
"I've done a bunch of them. I did a really great wolf's head. I did an amazing skull and crossbones. I did a stunning yin-yang on him. I messed every single one of them up," he added, laughing. "But he kind of likes it.
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"I did a yin-yang, which is probably one of the worst yin-yangs. We laughed so hard. It might be one of the worst yin-yangs ever made. A little bit egg-shaped."
Scott Campbell and his wife Lake Bell
Justin himself has several tattoos on his back and legs.
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"I'm riddled with them;" he said. "I'm ruined with tattoos. And Scott did most of the ruining, as well. He's done almost everything. I know Scott's taste is so much better than mine. He's going to come up with something better than I can come up with, so I usually throw out a few buckshot ideas, and then let him go back to the lab and scribble out a couple of drawings.
"He did my entire back, and I've never looked back. I was just, like, 'Do whatever the hell you want. You lay it out how you want.' He's got such an incredible aesthetic."