Demi Lovato & Sylvester Stallone Boxing

Demi Lovato's got hands of stone ... just ask Sylvester Stallone.

Demi and Sly strapped on the gloves Wednesday at Jay Glazer's Unbreakable gym on the Sunset Strip -- and she obviously has some ring experience ... of the Hollywood variety, at least.

Rocky was having fun, although he claims he slipped when he hit the mat. Decide for yourself.

BTW, the "Creed" sequel is slated for November, 2017. Get DL a script, stat!

Just sayin' ...