Renzi, 42, resigned as prime minister in December after Italians overwhelmingly rejected a constitutional referendum aimed at streamlining the parliamentary system.
Some two million people voted and Renzi himself had set the bar for success at one million voters.
Renzi's win yesterday was announced by him and his two rivals -- Justice Minister Andrea Orlando and Michele Emiliano -- well ahead of the end of counting.
According to preliminary estimates, Renzi bagged 70 percent of the vote.
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"This is not the second half of the same match, it's a new match," he said in a victory speech. "This is the beginning of a completely new story."
When he first took over the leadership in December 2013, Renzi won the backing of close on 68 percent of 2.8 million voters.
He managed to deliver significant labour market reforms and modest growth, while overseeing the granting of legal recognition to gay relationships for the first time.
Renzi then stepped down as party leader with the aim of regaining legitimacy in a future vote.
The winner of Sunday's vote will lead the party during legislative elections slated for spring 2018, unless parliamentarians come to an agreement on electoral reforms before then and call for early elections.
During the only televised debate between Renzi and his two competitors, the former prime minister said he would do "everything to bring back energy, momentum and vigour to the country", and railed against "stagnation that seems to be blocking political and institutional life" since the referendum.