The government will "continue to reform", Valls told the 3,500 delegates and observers at the congress in Poitiers, western France.
But he added that it will measure up to its responsibilities regarding employers, whose representatives signed President François Hollande's Responsibility Pact pledging to create jobs in exchange for tax cuts.
"We expect them to keep their commitments," Valls said, swearing that the government will bring down unemployment, which has stayed stubbornly high throughout its term so far.
Valls also promised to introduce at deduction at source for income tax, one of the pledges in the resolution that was passed in an electronic vote of Socialist Party members ahead of the congress.
He also laid into former president and right-wing opposition leader Nicolas Sarkozy, accusing him of waging personal vendettas and misrepresenting the left's policies.
"In politics you debate, you present your arguments," he told the party faithful. "You don't yell or insult. Through his behaviour in opposition Nicolas Sarkozy is already a problem for the country."
Representatives of the party's left-wing, whom Valls outmanoeuvred in parliament when they threatened to vote against the Responsibility Pact, took the stand on Friday to criticise the government for failing to deliver on past promises.
The party must "avoid the gap between words and deeds, between the motions we pass and the policy of the government", leading left-wing MP Christian Paul said.
The party rank and file voted in May for a resolution proposed by First Secretary Christophe Cambadélis and backed by Valls and most other government ministers.
The congress comes as the government struggles to tackle unemployment and revive its flagging popularity.
Opinion polls show less than 20 per cent of French voters approving of Hollande.
Valls scores higher - at 40 per cent in the latest - and most Socialist voters would prefer him as a presidential candidate in 2017.
He may not have the opportunity to stand, however, since the party is unlikely to hold a primary, as it did before the 2012 election, if the sitting president wishes to stay in the ring.