Mr Benvenuto, 55, received 307 votes at a party assembly against 223 for his only real rival, the Deputy Foreign Minister, Valdo Spini.
'I wish him luck - he and his party are going to need it,' said Marco Pannella, leader of the opposition Radical Party.
The Socialists, who count the Prime Minister, Giuliano Amato, among their members, form the second-biggest bloc in Italy's four- party ruling coalition. They have been battered by growing involvement in a huge bribery scandal and a disastrous showing in last April's general election.
Dozens of Socialists have been arrested on corruption charges and news last December that Mr Craxi himself was under suspicion provoked a deep split within the party ranks.
As judges sought to lift Mr Craxi's parliamentary immunity to prosecute him in connection with more than dollars 25m ( pounds 17.7) in suspect funds, the pressure on him to resign grew intense.
He finally stepped down on Thursday, after 16 years as unchallenged party leader, four of them as prime minister.
His main rival for the party leadership, Claudio Martelli, had also resigned as justice minister on Wednesday after being officially warned he too was being investigated. Mr Martelli was succeeded yesterday by Giovanni Conso, 70, a judge and former president of the Constitutional Court known for his strong defence of personal liberties before the law.
Mr Benvenuto became leader of the Unione Italiano del Lavoro (UIL), one of Italy's three main trade union groupings, in 1976. He resigned last year.
He was seen as the candidate most favoured by the party's old guard and may face criticism from the more ardent reformers who had until this week's crisis supported Mr Martelli.
In an acceptance speech Mr Benvenuto called for party unity and political reform. 'Everyone together - we have to take new responsibilities,' he said. Reform of Italy's electoral system and initiatives to combat unemployment would be party priorities, he added.
Initially regarded as a hardliner, Mr Benvenuto ended up with a reputation as a reformer and was jeered during last year's wave of union unrest by the militants he once championed. He is regarded as a close political associate of Rino Formica, a former finance minister who persuaded him last year to become the ministry's top civil servant. He still holds the post of director-general of the Finance Ministry.
When Mr Craxi stepped down on Thursday he said he wanted to devote his time and energy to defending himself against allegations that he was involved in the widening kickback and extortion scandal. While denying any wrongdoing, he has received six official notices that he is under investigation in connection with the scandal.
Socialists have been particularly hard hit by the scandal that has tainted politicians from all major parties. The alleged involvement of key party leaders and officials could weaken the leadership role of the Prime Minister, Giuliano Amato.
Mr Martelli, who denies any wrongdoing, was the first cabinet officer tainted by the scandal. News reports linked his name to a Swiss bank account the Socialist Party allegedly used to deposit money extorted in exchange for public-works contracts.