They say the alleged fraud amounted to $30 million.
Authorities told a news conference in Montreal this morning they seized 40,000 kilograms of tobacco and confiscated $450,000 in cash.
Police allege the Mafia bought the tobacco in the United States and imported it illegally into Canada through the Lacolle border crossing or through the Akwesasne Mohawk reserve.
It was then allegedly sold on the Kahnawake reserve south of Montreal.
Quebec provincial police, the RCMP, other police forces, customs agents and U.S. authorities were involved in the operation.