Ronnie and The Situation got into a MAJOR fist fight with each other last night in Italy ... TMZ has learned -- and based on the post-brawl pictures ... the Sitch got his ass kicked.
The two arrived separately back to their apartment hours ago -- Situation has red marks covering his face around his left eye. Ronnie's knuckles were torn up and bleeding.
Details around the fight are still unclear -- but big surprise ... we're told it all began with a blowout between Ronnie and Sammi.
Story developing ...