The elections will determine whether Macron, who was elected Sunday in a landslide victory over Le Pen, can win a workable majority to implement his platform of liberalizing the French economy.
Macron will now be looking to use his popularity to win hundreds of seats for his fledgling party En Marche! in legislative elections, in little over a month from now. He’ll be replaced by Catherine Barbaron.
Le Pen’s loss to centrist Emmanuel Macron still gave her a historic number of votes, reflecting the changing image of her once-pariah National Front party from fringe force to a political heavyweight.
Macron won 66 percent of the vote in Sunday’s presidential run-off against the far right’s Marine Le Pen, the biggest win by a French president since Jacques Chirac’s victory over Le Pen’s father Jean-Marie in 2002.
She said Mr Macron, who at 39 is to become France’s youngest ever president, carried the hopes of “millions” of French people, but also of “many” Germans, and she hoped the two would be able to tackle mass unemployment in Europe.
The former investment banker has little experience in governance, serving as economy minister for two years as his most senior role.
Macron has announced that he has picked his prime minister, but has not specified who it is. Another challenge will be to reach out to La France Profonde, the “forgotten France” of small, impoverished towns and villages that have been left behind as industries die and farmers struggle to make ends meet.
Monday was a French national holiday marking decades of peace in Western Europe, something Macron made a cornerstone of his campaign against Le Pen’s brand of nationalist populism.
Mr Valls said that “I will be a candidate in the presidential majority and I wish to join up to his movement, the Republic on the Move”.
Defeated rival Le Pen remains defiant.
Macron’s victory is a relief for Germany, the main defender of the European Union against rising anti-establishment sentiment on the right and left of the political spectrum.
President Hollande also called Macron to “warmly” congratulate him on his victory.
She was wrong, despite an early momentum in the first few days of the second round campaign that had Macron supporters anxious.
Valls, a center-leaning politician in favor of relaxing labor protections, had already thrown his support behind Macron before the presidential election after losing to Benoit Hamon in the Socialist primary.
Though Trump never endorsed Le Pen, in an interview to an American news agency he did say that he thought the attack would “probably help” her.
EU Council President Donald Tusk also offered his congratulations, saying the French had chosen “liberty, equality and fraternity” and “said no to the tyranny of fake news”.