French Socialist presidential candidate Segolene Royal has promised to embody change if she becomes France's first female head of state.
Ms Royal is said to have an even chance of winning
Ms Royal, 53, described her win over two rivals in a ballot of party members as "extraordinary", and said she wanted to write a new page in French history.
She won with more than 60% of the vote, avoiding a second round.
The mother-of-four is widely expected to face centre-right hopeful Nicolas Sarkozy in the April poll.
Ms Royal has promoted herself as a political outsider who can shake up a system with which many French people have grown disenchanted.
But BBC Europe editor Mark Mardell says she has so far been decidedly light on policy detail and faces a relentless campaign against Mr Sarkozy.
Speaking to reporters in Poitou-Charente, whose regional council she heads, Ms Royal expressed delight at the result.
"To be chosen in this way is something extraordinary," she said.
"I want to embody this change and make it credible and legitimate.
"I think that tonight this legitimacy has been given to me and for this I want to thank party members from the bottom of my heart."
Both rivals, former Finance Minister Dominique Strauss-Kahn and ex-Prime Minister Laurent Fabius, 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 Socialist rivals were involved in a bitter campaign
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.