The Hall of Fame wide receiver hit the dance floor at the Juniper Cocktail Lounge at Park MGM in Las Vegas over the weekend ... showing at 56 years old, dude still loves to get down!!!

Rice got behind the DJ booth while Pac's legendary diss track "Hit 'Em Up" blasted through the speakers ... dancing around and having the time of his life.

We're told Rice couldn't have been nicer with the partiers ... posing for pics and chatting for a while, before taking the rage fest to the On The Record speakeasy and club.

As you probably know, Rice LOVES to party -- he's a big hit in Lake Tahoe -- but the only difference here is he actually kept his shirt on this time around (classic video, BTW!!!).