A skilled communicator who models himself on US President Barack Obama and former British leader Tony Blair, 38-year-old Renzi is seen as a future prime minister but has promised to support the current government for now.
"We want Italy to change with pride in its past but also looking to the future," Renzi told a party congress that ratified his election to the leadership.
Renzi has pledged to slash the cost of Italy's bloated bureaucracy, cut down rampant youth unemployment and give more assistance to recession-hit business owners.
"If he cannot shake us out of this mess, then no one can," said Antonio Padellaro, editor of the Italian daily Il Fatto Quotidiano, adding: "After 20 years of useless words, this country can no longer wait".
The outgoing leader, Guglielmo Epifani, said Renzi's rise signalled "a generational turnaround" for the party, which has for years been dominated by middle-aged leaders.
The web-savvy Renzi, a moderate leftist, trounced his rivals in a leadership election earlier this month with 68 percent of the vote from members of the party.
Prime Minister Enrico Letta is also from the Democratic Party and was appointed earlier this year as a compromise choice to end a two-month deadlock following elections that were only very narrowly won by the party.
Speaking at Sunday's congress, Letta said: "Italy's exit from the crisis depends on the unity of our party."
Letta aims to stay in power at least until after Italy's European Union presidency in the second half of 2014, arguing that his left-right coalition can pass critical reforms and ensure political stability.