John Mayer unveiled his third nipple during his Instagram Live chat show on Sunday evening.
The 'New Light' hitmaker recently relaunched his 45-minute talk show, 'Current Mood with John Mayer', on his social media account, and Mayer was joined by 'Watch What Happens Live' host Cohen, 50, as his guest of the week.
However, the pair got sidetracked by the singer's "third nipple" and how it differs from his two regular nipples.
Speaking on 'Current Mood', Mayer, 41, confessed: "I have three nipples. This is a Current Mood first."
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."