François Fillon was voted the presidential candidate of the French centre-right party and will likely be the main challenger to far-right leader Marine Le Pen in next year's election.
Votes from over a third of 10,228 polling stations were counted showing Fillon winning with 68.6 per cent of the vote compared with 31.4 per cent for Alain Juppe, his rival.
Juppe conceded defeat and congratulated Fillon on a "wide victory".
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"I must now convince the whole country our project is the only one that can lift us up," Fillon said at his campaign headquarters.
Fillon, 62, is a socially conservative free-marketeer and former French prime minister.
Last week, Fillon knocked out former president Nicolas Sarkozy, under whom he served as prime minister from 2007 to 2012.
The Socialist primaries are set for January, and the unpopular current President François Hollande has two weeks in which to decide whether to take part and run for re-election.
Opinion polls suggest neither Hollande nor any left-wing candidate would make it through the second round of the election in May, leaving Fillon to run against the anti-EU, anti-immigration National Front leader Le Pen. Surveys predict he will win.
The election will likely be another test of a Western country's anti-establishment anger.