The 32-year-old man was found in a pool of blood with a deep head wound after passersby reported a fight, police said in a statement.
As he was rushed to hospital, suffering from head trauma and multiple fractures, police followed up on claims that he had been attacked by a local restaurant owner.
On entering the eatery, police found a broken beer bottle, a samurai sword and a baseball bat.
The 40-year-old owner and his 45-year-old wife, both from the Dominican Republic, then confirmed that they had attacked the man because he left without paying for his beer. The pair were promptly arrested, police said.
The attack comes weeks after an unprovoked attack in Rome using another Japanese weapon. In December, a pensioner’s throat was slashed with a ninja star, a multi-pronged Japanese weapon, in the popular Trastevere neighbourhood.