Promoting his new album, "Believe," Bieber showed off some new ink on his left arm.
But merely looking at the Bieb's tattoo wasn't enough for Letterman, who grabbed the teenager and started trying to rub it off.
He then offered Bieber some advice, telling him to be careful not to end up covered in tattoos and looking like "the Sistine Chapel ... it's too much."
"I’m not going for the Sixteenth Chapel look," said Bieber. "Sistine Chapel," corrected Letterman. "Sixteen," argued the singer.
Later on, Letterman apologized for the way he treated Bieber, saying he'd been told he was "snappish" with him.
"Late Show" airs weeknights at 11:35 p.m. ET on CBS.
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