Jennifer Lopez Reveals ID of Creepy Guy in Background of Her Gym Selfie

Jennifer Lopez finally dished on a mystery people have been trying to ID for weeks now -- revealing the creepy floating face that popped up in one of her latest gym selfies.

She made the revelation on a recent appearance on 'The Tonight Show' with Jimmy Fallon, eventually getting past the pleasantries and asking what America wanted to really know ... WHO THE HELL WAS THAT GUY WITH A HAND OVER HIS MOUTH?!?

The answer's not as salacious as some might've expected -- J Lo says it was just a dude her fiance, Alex Rodriguez, was talking to on Zoom on the other side of their gym window.

We'll let Jen explain for herself here, but basically ... with the way they have their gym set up, and where they have one of their laptops perched for Zoom chats, the two just happened to overlap and captured this guy A-Rod was chatting up in a very scary pose.

She doesn't know for sure who exactly it was -- other than the fact that he's in real estate -- or why he had his hand over his face (might've been a sneeze or a cough) ... but the good news is that it wasn't some rando creepin' on Jennifer from behind. That's low-key what some folks thought it was at first, and thank God that didn't turn out to be the case.