The French premier has declared his candidacy for the 2017 presidential election. As a result, he will step down from his current role.
The 54-year-old announced on Monday that he would be a candidate for January's primaries to choose the Socialist nominee.
"Yes, I am candidate for the presidency of the Republic," Valls said in a speech to supporters in the commune of Evry, south of Paris. "I want to give everything for France."
The declaration comes after his party colleague, President Francois Hollande, last week said he would not seek a new term. Valls said he would quit as prime minister on Tuesday, adding that he had the blessing of Hollande.
Opinion polls make Valls favorite to win his party's ticket, although his pro-business reforms and tough stance on law and order have earned him enemies within his own party.
Valls said he wanted to bring together the divided French left, and appealed for its members to unite behind him. "My candidacy is one of conciliation, of reconciliation," Valls said.
Facing an uphill battle
Even if he secures the nomination, surveys show the presidential contest is likely to come down to a run-off on May 7 between conservative Francois Fillon and Marine Le Pen, leader of the anti-EU, anti-immigrant National Front.
Valls has been Hollande's prime minister since March 2014, after being his interior minister for two years. Although he had strong opinion poll ratings at that time, these have since suffered.
Former Economics Minister Arnaud Montebourg and the ex-education minister Benoit Hamon, part of a group of Socialist rebels, have announced their own candidacies.
rc/se (AFP, dpa, Reuters, AP)