Francois Hollande has been elected the French Socialist Party's candidate for the 2012 presidential vote, where he is expected to face Nicolas Sarkozy.
His rival, Martine Aubry, conceded victory in primary run-off on October 16 before final results were known.
In his victory speech, Mr Hollande said he would champion those who could "no longer bear" Mr Sarkozy's policies.
Opinion polls suggest he could beat incumbent President Nicolas Sarkozy, who has not formally declared his own bid.
Mr Hollande had a lead of 56% to 44% over his rival in the count.
It is the first time voters have taken part in a US-style primary race, and some 2.7 million voters took part in the run-off.