Bilel Chiahoui, 26, was arrested on Thursday after posting messages on social media praising those behind the jihadist attacks in Europe and saying he would attack the Tuscan city's famous monument, a huge tourist attraction.
His expulsion order was signed by a judge on Friday according to a police report, which said there was "evidence the Tunisian sympathised with jihadist extremism and IS," a reference to the Islamic State group.
The police statement did not provide details on the target of the suspected attack.
There has been mounting concern about the possibility of Italy being hit by "lone wolf" terror attacks of the kind seen in neighbouring France and Belgium.
Recent weeks have seen a string of suspected Islamists expelled from the country on the order of Interior Minister Angelino Alfano.