It might be sunny down at Major League Baseball Spring Training in Florida, but the moon came out early during one game -- 'cause a mischievous fan pulled down his pants and mooned the camera!

The Toronto Blue Jays were taking on the Philadelphia Phillies Friday at TD Ballpark in Dunedin, FL ... and, one fan behind the plate appeared to be talking to a pal on the phone.

This isn't abnormal at baseball games -- where the fans behind the plate are in full-view of the audience at home ... but, most of the time these fans give a wave or a thumb's up to the folks at home.

This Blue Jays fan decided to take it a step further ... unbuckling his pants, turning around and showing his crack to everyone watching.

Watch the clip for yourself ... the woman with him is clearly laughing while trying to stop him -- though we can't imagine everyone at home was happy to see this Jays fan's tail feathers.

BTW ... the Blue Jays won Friday -- though the exhibition games have no bearing on the standings -- so, maybe this fan's butt is actually a good luck charm?

We've reached out to Major League Baseball and local police to see if any investigation is underway ... so far, no word back.