Segolene Royal will be the French Socialists' presidential candidate at next year's election, after beating two rivals in a ballot of party members.
Segolene Royal took 60% of the vote in the primary race
The 53-year-old mother-of-four hopes to become France's first woman leader.
She won a clear victory over former Finance Minister Dominique Strauss-Kahn and ex-Prime Minister Laurent Fabius, avoiding a second round of voting.
Ms Royal's main opponent from the right is widely expected to be Home Affairs Minister Nicolas Sarkozy.
"To be chosen in this way is something extraordinary," Ms Royal told reporters in the western region of Poitou-Charentes.
"I want to embody change and give it credibility and legitimacy. And I think today, I have received this legitimacy."
Both her rivals conceded, saying it was important for the Socialists to forget their differences and focus on the election, which has been set for 22 April.
Mr Sarkozy has not officially announced that he is a candidate.
Recent polls have suggested Ms Royal would stand an even chance of beating Mr Sarkozy in the presidential poll - but her two rivals would not.
The three contenders made bitter attacks on each other during the campaign.
If no candidate had achieved an overall majority of votes, there would have been a second round next week.
But party officials said 60.6% of the party's 220,000 members had plumped for Ms Royal, a former environment minister.
Mr Strauss-Kahn took 20.8% of the vote and Mr Fabius 18.5%.
Ms Royal's rivals tried to destabilise her campaign during the final days on the election trail.
Teachers reacted angrily after a video appeared on the internet last week featuring a party meeting earlier this year in which she said they should work longer hours in school.
Ms Royal responded by highlighting what she described as "chauvinistic comments" made by her rival candidates, something they strongly denied.