Councillor Massimo Adriatici was put under house arrest after a scuffle in a public square on Tuesday night in the northern city of Voghera, according to local news media.
The politician, who was holding a pistol, claimed he was shoved by the man and a shot went off as he fell to the ground and hit the victim in the chest.
Italian news agency Ansa said Adriatici had been arrested on suspicion of murder, but prosecutors were also looking into a potential charge of excessive self-defence.
Local media said the victim, a 39-year-old Moroccan man, had been harassing people outside a bar.
Police did not respond to an AFP request for comment.
"Victim of an aggression, he responded accidentally with a shot that unfortunately killed a foreign citizen," said Salvini in a video posted on his Facebook page, calling the incident "legitimate defence".
Salvini -- a former interior minister -- and his party have long been criticised by the left for their anti-immigrant rhetoric.
Alan Ferrari of the centre-left Democratic Party called Adriatici's actions "serious and worrying", and urged Salvini to condemn it.
"In a civil and democratic country, a councilman doesn't shoot a person," Ferrari said.
