On last night's US chat show, the 16-year-old U Smile singer plugged David's programme, although Letterman himself wasn't sure what Justin's tweet would accomplish.
Bieber asked: 'You want me to Twitter you?'
David responded: 'What do you mean Twitter me?'
Justin added: 'I will just tweet, like just say you're here. It's just cool to let the fans know.
'I just said I'm hanging out with my boy Dave.'
Bieber then tweeted: 'I'm hanging out with my Dave at @late_show we need to get him twice as many followers!'
Justin then further confused the 63-year-old by telling him that he was in the 11th grade at school.
Letterman asked: 'What is 11th grade? Is that sophomore in high school?'
Bieber responded: 'I don't know, I am Canadian.'
At one point, Bieber wore some 3D glasses as he appeared on the show to promote his 3D concert film Never Say Never and he also gave a lucky audience member some pudding.