Cohen quickly replied: "No, this is a world exclusive!"
The "New Light" chart topper then lifted up his shirt to show off what he called a "supernumerary nipple."
Whilst touching his third nipple, Cohen quipped: "It kind of is."
Mayer added: "You're the worst doctor ever! I showed you something with a medical name and you kept going, 'It might be!'"
Cohen joked: "I'm no doctor, obviously, but is it a mole of some sort?"
Mayer - who recently announced his solo 2019 world tour alongside a second with Dead & Company - also confessed that all three of his nipples have no "sensitivity".
Cohen joked: "Do you feel marginalised as someone with a third nipple? I feel like that would be hard sometimes."
Mayer - who previously dated Katy Perry, Jessica Simpson and Jennifer Aniston - replied: "I'm kind of doing alright, but some people are. I'm a lucky one. I'm a well to do three nipper."
Cohen asked: "Is your third nipple sensitive?"
Mayer said: "No, but my first and second aren't either."