In fact, the 35-year-old was spotted rocking out to Def Leppard's resident show VIVA Hysteria! at The Joint at Hard Rock Hotel & Casino in Las Vegas, where the English rock band is currently headlining for an exclusive residency.
The band performed their iconic album Hysteria in full along with a selection of their greatest hits. John is clearly a huge fan as he hung out with band backstage and posed for pictures.
Here's hoping we get a cover version of "Pour Some Sugar on Me" on Mayer's next CD.