Elected leader of the Democratic Party (PD) in December, the photogenic former Boy Scout is liked because he has never been in government or parliament on a national political scene tainted by corruption scandals.
A poll last month found 54 percent of Italians liked his leadership style, a much higher proportion than the roughly 25 percent who would vote for his party.
On Thursday he said there was "necessity and urgency" for a new government in Italy in his first direct challenge to Prime Minister Enrico Letta.
With his catchphrases and hashtags and an informal style, he is particularly popular among younger voters.
He often cycles around Florence and can be seen wearing tight jeans, leather jackets and retro sunglasses
For all his outward appeal, Renzi also has many critics who point to his lack of government experience as a crucial flaw and say that his first moves at the leadership of the party have shown an arrogant style.
Renzi's meeting with disgraced former prime minister Silvio Berlusconi last month was a case in point.
Despite opposition from many leftists, Renzi went ahead with the talks and struck a deal with Berlusconi on political reforms that would lessen the power of smaller parties in Italy's electoral system.
Renzi brushed off his critics and the president of the party Gianni Cuperlo resigned, accusing the party secretary of not being able to take criticism.
Ever since his rise, Renzi has been at the centre of media attention in Italy but is still little known internationally - although he has said he looks to former British prime minister Tony Blair as a model.
Renzi has pushed for more cuts in spending on Italy's unwieldy bureaucracy amid widespread anger over high salaries for public officials even during a painful recession, as well as a greater focus on education.
With his mamma's boy good looks, he is considered a politician with "transversal" appeal who can win over part of the centre-right electorate and has even been spoken highly of by Berlusconi.
Another link with Berlusconi is that one of Renzi's main spin doctors is Giorgio Gori, a former director of Berlusconi's main television stations.
Advocate of 'New Labour'
Born on January 11th 1975, in Florence, Renzi studied law and had his first brush with politics at the tender age of 19 as he followed in the footsteps of his father, a local Christian-Democrat party politician.
In 1994, Renzi created a committee to support Romano Prodi as he vied to lead Italian centre-left forces against Berlusconi.
He then worked for a few years at a marketing company that belongs to his family and makes most of its money thanks to a local Florentine paper, La Nazione.
Renzi's leap into politics proper came in 2001 when he became a local organizer for La Margherita, a Christian centre-left party.
He was selected by the centre-left to run in elections to lead the province of Florence in June 2004 and won with 58.8 percent of the vote.
Renzi became an advocate of political renewal along the lines of "New Labour" in Britain and earned popularity by lowering local taxes, establishing an efficient recycling system and promoting culture and innovation.
It was only his mayoral victory in Florence, however, that attracted wider national attention.
He clinched a surprise win in a primary against another more favoured candidate from the Democratic Party thanks to his media skills and his grassroots campaign on the squares and in the markets of the historic city. He went on to win the mayor's seat in 2009.
Before his first unsuccessful campaign for the party leadership in 2012, Renzi attended the Democratic National Convention in the United States and spoke there of his deep admiration for President Barack Obama.
He won the nomination in a repeat primary in 2013 after an inconclusive general election in which his predecessor Pierluigi Bersani was widely criticized.
Renzi is married to a former fellow Scout, Agnese, a schoolteacher, and the couple have three children.