Mr Renzi needs to seal a formal coalition deal with the small centre-right NCD party to secure a majority and to name his cabinet before seeking a formal vote of confidence in parliament, probably later this week.
He has promised a radical programme of action to lift Italy out of its most serious economic slump since the second world war, but will have to deal with the same unwieldy coalition which failed to pass major reforms under its previous leader.
After a 90-minute meeting with President Giorgio Napolitano yesterday, at which he was given a mandate to form a new government, Mr Renzi said: "In this difficult situation, I will bring all the energy and commitment I am capable of.
"The sense of urgency is extraordinarily delicate and important but it's also true that, given the time horizon we have set of a full parliamentary term, we'll need a few days before formally accepting the mandate."
The 39-year-old mayor of Florence, who is popular with the public, had been expected to take over since Prime Minister Enrico Letta stepped down last week after the Democratic Party leadership backed a call for a new administration following growing impatience with the slow pace of economic reforms.
Italy, the eurozone's third largest economy, is technically no longer in recession since it scraped back into growth in the final quarter of 2013.
However, it remains profoundly marked by the crisis, with massive public debt, a rapidly crumbling industrial base and millions out of work.
Mr Renzi said he expected to lay out full reforms to Italy's electoral law and political institutions by the end of the month, to be followed by labour reforms next month, an overhaul of the public administration in April and a tax reform in May.
He made no comment on the likely make-up of his cabinet.