Valls announced his candidacy in a speech from his political base in the tough Paris suburb of Evry.
French Prime Minister Manuel Valls on Monday threw his hat in the ring to succeed Francois Hollande in next year's presidential election.
"I am a candidate for the presidency of the Republic," Valls said, announcing he would step down as premier on Tuesday to campaign for the Socialist nomination in a primary in January.
The 54-year-old Spanish-born politician announced his candidacy in a speech from his political base in the tough Paris suburb of Evry.
"I have a responsibility to unite," he said, appealing to the fractured left to rally behind him.
He warned of the risk of far-right leader Marine Le Pen pulling off a repeat of France's 2002 electoral upset when her father Jean-Marie Le Pen edged the Socialist candidate for a place in the presidential runoff.
Le Pen, he said, would "take us out of Europe" and her policies would "ruin the working class".
Polls show the younger Le Pen winning or being placed second in the opening round of the election on April 23 but later being defeated by the conservative Francois Fillon.
But with Sunday's Italian referendum defeat for Prime Minister Matteo Renzi just the latest rout of mainstream political figures at the ballot box, no-one is ruling out a Le Pen victory.
Valls said his candidacy was "a revolt against the idea that the left has been disqualified".
His entry into the race had been expected after Hollande announced last week he would not seek a second term, bowing to pressure to step aside after a troubled four years.
Health Minister Marisol Touraine and Interior Minister Bernard Cazeneuve are among those tipped to succeed Valls as premier for the six months until the legislative elections that follow the presidential vote.
The divided French left currently appears ill-equipped to retain power.
Valls is a polarising figure among Socialists, accused of being a closet conservative for using decrees to force labour reforms through parliament and endorsing controversial bans last summer on the Islamic "burkini" swimsuit.
He also faces an uphill battle to defend his government's bleak economic record.
In his first campaign speech Monday he argued he had tried to make France more competitive while defending its cherished social model.
He faces a challenge from at least seven other candidates in the two-round primary on January 22 and 29.
Martine Aubry, a veteran Socialist and the mayor of the northern city of Lille, said Monday she was not certain she would back him.
Polls show Valls nonetheless winning the nomination but trailing in the presidential race, held back by competition on the left from business-friendly former economy minister Emmanuel Macron and the fiery hard-left Jean-Luc Melenchon.
When Valls last sought the Socialist nomination five years ago, he garnered only 5.6 percent on a platform seen as too economically liberal.
He went on to become a spokesman for Hollande's campaign and when the Socialist won the presidency was rewarded with the interior portfolio.
In 2014, the tough-talking minister was promoted to premier with a mandate to rein in a group of unruly ministers who were undermining Hollande's authority.
Within months he had a rival for the title of reformer-in-chief, in the telegenic banker-turned-economy minister Macron.
Hollande's protege walked out on the government in August to further his own presidential ambitions.
In a newspaper interview last month, however, Valls had himself turned on Hollande, saying explosive revelations contained in a book of interviews between the president and two journalists had "plunged the left into total disarray".