The famous ladies' man reportedly regularly meets up with another lady for illicit trysts.
The American singer is currently dating Katy Perry, with the pair going public with their romance in New York City last week.
Although it's thought Katy is serious about her new beau, it's been suggested John might not be so into her. Mayer who also dated Jessican Simpson and Jennifer Aniston is rumoured to be seeing another woman on secret hook-ups.
"They meet frequently, just for sex dates," an insider told Life & Style. "And they sext [sexy text message] each other almost constantly."
According to the publication, John and the unknown woman have been seeing each other since early in 2011.
Sources are concerned about the effect the information could have on Katy. She started dating John following her divorce from Russell Brand and was reportedly worried about his womaniser reputation, only getting serious with the singer once she thought he felt the same.
"She’s totally into him," the insider said. "There’s a part of her that thinks she’s definitely reformed him, so she’d be totally devastated if she knew that John was cheating on her.
"John and this woman have incredible sexual chemistry and have been seeing each other nonstop since their first encounter almost two years ago."
In 2010, he hit the headlines after giving an interview to Playboy in which he graphically discussed his romance with Jessica.
"That girl is like crack cocaine to me," he said. "Sexually it was crazy. That's all I'll say. It was like napalm, sexual napalm.
“Did you ever say, 'I want to quit my life and just f**king snort you?'"