Ex-education Benoit Hamon minister comfortably beat Manuel Valls, a former prime minister
Benoit Hamon will be the French Socialist Party's candidate in the presidential elections in April, after winning a run-off vote on Sunday.
The ex-education minister comfortably beat Manuel Valls, a former prime minister, who conceded before the final tally was completed.
With 60% votes counted in the Socialist run-off, Mr Hamon had just over 58% to Mr Valls' 41%.
After his win was announced, Mr Hamon said, "Despite the differences, the forces of the left have never been so close in terms of ideas. Let's come together."
He called on the Socialist Party, independent left-winger Jean-Luc Melenchon, and a Green candidate to unite and "construct a government majority".
Mr Hamon, 49, was the most left-leaning of the seven initial candidates in the Socialist race, the first round of which was held last week.
He has experienced a surge in popularity from a range of progressive plans, including a proposal for a universal monthly income for all citizens.
He also wants to legalise cannabis, and ditch the labour law passed last year that made it easier to hire and fire.
Despite the victory, the Socialists are not expected to do well in the election due to outgoing President Francois Hollande's low approval rating.
Conservative Francois Fillon, right-wing Marine Le Pen, and centrist Emmanuel Macron lead the polls.