Bill Belichick's Biggest Secret? Dude Can Sing, Says Girlfriend

Bill Belichick is secretly the Bill Belichick of music -- so says his longtime girlfriend who just revealed the Patriots coach is a closet SINGING SENSATION!

Linda Holliday dropped the musical bombshell in an interview with N Magazine, saying:

"I’ll tell you something about Bill ... He can sing. And he can sing well."

She noted, "You won’t hear it. You won’t see it. But he can sing well.”

Belichick -- who was sitting next to Holliday for the interview -- reportedly busted out a "boyish grin" when Linda spilled his secret.

So, we did some digging and actually FOUND a clip of Bill's pipes in action -- singing with Jon Bon Jovi!

Look, he ain't exactly Pavarotti ... but cut the guy a break -- he was probably just a little tired from WINNING SUPER BOWLS!

One more note, the interview was to raise awareness for the Bill Belichick Foundation which helps kids through sports.