The Riva family that owns the company is under multiple investigations for fraud and toxic pollution at its main plant in Taranto in southern Italy.
It said it had to close the other plants too because of a decision by prosecutors to freeze its assets and current accounts with €916 million.
Trade unions staged protests against the closures.
Economic Development Minister Flavio Zanonato was quoted by La Repubblica daily as saying he did not know why Ilva had decided to shut its plants.
"There should be no doubt that this can have a knock-on effect and other companies could find themselves in difficulty," Zanonato said.
Prosecutors in the city of Taranto went further, defining Ilva's decision as "blackmail", according to judicial sources.
The steel sector as a whole in Italy - which is dominated by Ilva and the Riva family - employs around 50,000 people.