François Hollande won the second round of primary elections against Martine Aubry, Socialist Party leader and mayor of Lille, with more than 57% of the vote.
The Socialist Party had organised a two-round contest where anyone who paid a euro and declared allegiance to left-wing values could vote.
Aubry, the daughter of long-serving Commission President Jacques Delors, saluted the winner, saying the time had come for unity of the French left to win back the presidency after ten years of centre-right rule and "five years of Sarkozyzsm".
The only Socialist President in post war France was François Mitterrand, who served from 1981 to 1995.
Hollande, 57, is little known outside France. For many years, he has been the partner of Ségolène Royal, another candidate who lost the first round of the primary election and is the mother of their four children. Royal lost against Sarkozy in 2007.
The left's favourite candidate for the election was former International Monetary Fund chief Dominique Strauss-Kahn. However, both his IMF career and presidential hopes foundered when he was arrested in New York in May on charges of sexually assaulting a hotel maid. The charges have since been dropped.
In contrast with flamboyant Sarkozy, Hollande is seen as "Mr. Normal", a person wearing inexpensive suits and riding through Paris in a three-wheel motorcycle. In contrast with other Socialist contenders, including Arnaud Montebourg, who had advocated protectionism and bank nationalisation, he is seen as "a moderate" who does not aim at unsetting markets.
"I would like the candidate of respect, of dialogue and of democracy," he said in his speech following the announcement of his victory yesterday night.
In his campaign, Hollande said he had heard the worries of the French people over financial disorder, the excess of globalisation, the shortcomings of Europe and the many threats to environment.
Friends of Hollande said he would soon start a campaign to build for himself an identity abroad.
"Foreign chancelleries want to meet him. They do not really count on a Sarkozy reelection," a friend of Sarkozy was quoted by Reuters as saying.
