Johnny Manziel -- Allegedly Punched Man at Cleveland Hotel ... Police Investigating

Johnny Manziel allegedly punched a man in the face in a huge brawl at a Cleveland hotel Friday night and ... and cops tell TMZ Sports an investigation is under way.

According to a police report filed with the Cleveland PD, a man named Chris Gonos claims he approached Manziel outside of an elevator at the The 9 hotel late Friday evening.

Gonos tells TMZ Sports he wanted to give Manziel a handshake and a hug -- but someone in Johnny's entourage wasn't havin' it and tackled him.

Gonos says a melee ensued ... and claims Johnny "sucker punched my brother in the back of the head."

According to police, hotel security broke up the fight. No arrests were made.

Cleveland Browns General Manager Ray Farmer released a statement Monday, saying, "We are aware of the incident and are in the process of gathering additional information in order to gain a complete understanding of what occurred."