The 27-year-old singer went to a late-night house party with Mayer, 34, and fuelled runours that she is dating him, reported Femalefirst.
"John has a big reputation as a serial womaniser. If people thought her ex, Russell Brand, was bad, John is notoriously a lot worse.
"Katy's friends are worried this is just a rebound bit of fun but will end with her getting really hurt," a source said.
Mayer has previously dated Jennifer Aniston, Cameron Diaz, Jennifer Love Hewitt, Kim Kardashian and Taylor Swift 22, among others.