Fiat "will immediately launch the investment plan needed to ensure the production and labour future of the Mirafiori plant," chief executive Sergio Marchionne said in a statement.
In return for the investment, trade unions said they would support the collective contract proposed by the company which toughens work conditions.
The unionists said the contract was "an unavoidable condition for Fiat's commitment to Italy".
But the Fiom-Cgil metal workers' union, which is in a long-running dispute with Fiat, did not agree to it.
Luigi Angeletti, leader of the Uil trade union, said the investment would allow work on ensuring production of the 4x4 Maseratis at the plant to start within weeks.
"We will have to change our production line to produce a very different type of car," he said, adding that the actual cars would be built from 2014.
Giuseppe Farina, another trade unionist from Fim-Cisl, said it was "good news for Fiat employees and for Italian industry".
"This returns future and hope to many employees of the company and to the country's auto industry," he added.
Fiat is Italy's biggest private sector employer and is regularly accused by critics of planning to leave Italy following its merger with US partner Chrysler.