Venizelos triumphed Sunday in his bid to become president of PASOK, reportedly winning 189 votes out of the 377 members of the party's National Council.
The finance minister will take over the party from George Papandreou, who resigned as prime minister on 11 November 2011 to make way for a national unity government charged with tackling the Greek government debt crisis.
"We did not just avoid default, we changed the way that the whole planet sees us," Venizelos said in his acceptance speech.
"The crisis is not over, the key is work, work, work," Venizelos said after thanking the citizens for their sacrifices and apologising on behalf of the political system.
In the post-dictatorsip era after 1974, Greece has been alternatively governed by two big parties, the Panhellenic Socialist Movement (PASOK) and the centre-right New Democracy party, currently led by Antonis Samaras.
PASOK founder Andreas Papandreou and his son George have governed overall for more than 20 years, while from the centre-right Konstantinos Karamanlis and his nephew Kostas Karamanlis have been in power for most of the remaining time.
The next step is a poll on 18 March in which PASOK supporters will be asked to pay €2 to cast their ballots at town halls around the country, even though Venizelos is the only candidate for the party leadership.
"I am fully aware of the responsibility that I am taking on," said Venizelos, a constitutional law professor. "But this responsibility is collective and we will act as a team."
A recent Public Issue survey for the Kathimerini daily suggested that support for the Socialists was at just 11%. But this did not deter Venizelos, who claimed that the center-left party could win the general elections.
New Democracy continues to lead in public opinion with 28% of the vote, but not enough to form a governing majority in the 300-member parliament even with a 50-seat bonus awarded to the leading party under Greece's electoral system.
Surveys show that voters continue to throw their support behind smaller, fringe parties as support for mainstream politicians withers.