Macron, 39, was elected France's youngest-ever president on Sunday, crushing far-right leader Marine Le Pen after a bruising campaign that left France's traditional parties by the wayside.
He faces a huge task to unite a fractured, anxious country and to win a parliamentary majority in June's general election, without which he could struggle to implement his ambitious reform agenda.
Macron's victory at the head of a year-old movement that has presented itself as a home for progressives of all stripes has blown up France's long-standing left-right political divide.
"This Socialist Party is dead, it is behind us," said Valls, a reform-minded premier from 2014 to 2016 when Macron was economy minister.
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"I will be a candidate for the presidential majority," the 54-year-old Valls told RTL radio, while insisting he remained a Socialist and "a man of the left".
Macron's "Republique en Marche" (Republic on the Move) movement, which has been keeping its distance from the political old guard, reacted warily to the announcement.
Socialist leader Jean-Christophe Cambadelis attempted to prevent a wave of defections to Macron's camp, warning Valls he could not remain a party member and run for parliament on Macron's ticket.
"That's impossible," he said.
A relative newcomer to politics, the europhile Macron swept to victory with 66.1 per cent of the vote on a tide of opposition to Le Pen's anti-immigration, anti-EU platform.
He has promised to liberalise the economy and rejuvenate France's jaded governing class by bringing more people who, like him, have never held elected office into government and parliament.
The rest will be from the centrist Modem party or rebels from the Socialists and right-wing Republicans. The candidates will be announced by Thursday.
The losers of the presidential election are aiming to capitalise on a general lack of enthusiasm for the pro- business Macron -- whom many voters backed solely to bar Le Pen -- to bounce back in the parliamentary vote.
The Republicans, Le Pen's National Front and Socialists all held meetings today to discuss strategy.
Republicans grandee Alain Juppe said the party should not adopt a policy of "systematic obstruction" of Macron if it failed to win a majority.
Macron, who will be inaugurated on Sunday, takes over from his former mentor, the deeply unpopular Socialist President Francois Hollande, who decided against seeking a second term.
One of Macron's first acts will be to name a prime minister -- a choice that will show whether he leans more to the left or the right.
"Macron carries the hopes of millions of French people and also many in Germany and across Europe," German Chancellor Angela Merkel said.
But Le Pen's historic score of 33.9 per cent, or 10.6 million votes, showed the far right to be a formidable force in a country wracked by concern over immigration, national identity and globalisation.
The fervently pro-European Macron wants to team up with Germany to reform the EU, which has been weakened by Britain's vote to leave.
Commission chief Jean-Claude Juncker has appealed to him to mind the public purse.
"We have a real problem with France," Juncker said in Berlin yesterday. "The French spend too much money and spend it on the wrong things.
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