Barack Obama Taps Michelle's Butt at the Acropolis in Greece

Barack Obama is taking in one of the most hallowed sites in ancient Greece ... but he's clearly more impressed with his wife Michelle's rear end! Smart man.

The former President apparently couldn't resist playfully giving Michelle a helping hand Wednesday during a stop at the famous Acropolis in Athens, palming her booty as they walked up a staircase together at the venue.

As you can see, Barry O has his left hand on Michelle's right cheek as she climbs the stairs in front of him. Their daughters, Malia and Sasha, were nearby ... hopefully they didn't have to witness Dad's PDA play.

Barack, Michelle and the girls are in Greece for an Obama Foundation event ... and it looks like they took a little time out from their business trip to do the tourist thing.

Based on what we're seeing here, it seems Barack and Michelle still have that spark. They first met way back in the summer of 1989 when they worked at the same law firm, so this fire's been burning for decades.