The former mayor of Florence was accompanied by his wife and three children - dressed in the colours of the Italian flag - to the formal ceremony in Italy's presidential palace, and smiled widely as he watched his new team sworn in by President Giorgio Napolitano.
The centre-left leader takes over the reins of the eurozone's third largest economy in a period of increasing frustration among ordinary Italians hard hit by a deep recession and weary of broken political promises.
Renzi is "gambling on freshness, newness and energy", but "doubts must be raised over the government's experience and ability to have a bearing on the worst post-war economic crisis Italy has known," said political watcher Mario Calabresi in La Stampa daily.
Il Sole 24 Ore financial daily warned that "Italians are waiting for reforms, not just pretty smiles."
More From This Section
European partners will be watching closely to see whether the 39-year-old can revolutionise the halls of power after ousting his predecessor Enrico Letta for failing to enact reforms in a country often perceived as stifled by corruption and bureaucracy.
"Renzi seems to be betting everything on himself, on his political energy," editorialist Ezio Mauro for La Repubblica daily said, while Marco Travaglio in Il Fatto Quotidiano described the new cabinet as "a boiled chicken soup which disappoints even the most lukewarm expectations.