The suspect, named by Ansa as 23-year-old Elmahdi Halili, was detained in the north-western city on suspicion of "participating in the terrorist association Islamic State [Isis]".
The Moroccan-Italian is said to be the author of the first piece of Italian-language Isis propaganda.
Police believe he hoped to organize attacks in Italy using trucks and knives, and may have tried to recruit "lone wolves" to carry them out.
"It was time to intervene. We couldn't afford to wait until he identified a target to strike," said the head of Turin's special operations police squad, Carlo Ambra.
Halili is known to have contacted a number of people, both Italians and foreign nationals, some of them already known to police, in what investigators believe was an attempt to recruit them for acts of terrorism.
A police handout of Elmahdi Halili. Photo: Polizia di Stato/AFP
Police conducted a total of 13 searches as part of Wednesday's operation, raiding premises in Milan, Bergamo, Modena, Reggio Emilia and Naples.
Several other people are reported to be under investigation, accused of attempting to radicalize others online.
Halili has already been convicted of terror-related charges once before, in 2015, when he was found to have published Isis texts on the internet.
He was sentenced to two years in prison as part of a plea deal. After his release police found that he continued to publish material glorifying terrorist acts, including videos of Isis violence and its militants in Syria and Iraq, as well as playlists of speeches by infamous radical Islamists.
"He's a highly motivated individual with no intention of reforming," Ambra said.
As officers led him away on Wednesday, he reportedly shouted: "Tyrants! I'm going to prison with my head held high."
His arrest comes the day after a separate operation that saw an Egyptian-Italian man detained in the southern city of Foggia, where he was accused of attempting to indoctrinate youngsters at a Muslim cultural centre.
Police are also investigating a teacher from the city now working in Ferrara, an Italian citizen who converted to Islam and who is believed to have been in contact with the man arrested, it was reported on Wednesday.
Though neither man arrested this week was in Italy illegally and both are Italian citizens, the leader of the populist right League party, Matteo Salvini, called on Wednesday for tighter controls of Italy's borders and an immediate halt to migrant arrivals by sea.
He referred to comments from the European border agency Frontex, which said this week that unregulated crossings from North Africa into Europe raised security concerns, amid a level of terrorist threat that remains high.