Hamon, 49, shot up in voter surveys during a rushed campaign that saw three debates in little over a week in January.
Hamon, 49, whose signature proposal is the introduction of a universal basic income, shot up in voter surveys during a rushed campaign that saw three debates in little over a week in January.
His key proposal, which would see unemployed and low-income workers receive a monthly payment of between 600 and 750 euros ($795) a month, would entail a costly and radical reform of state spending -- some 300 billion euros a year, by Hamon's own estimates.
He sees basic income as a response to the erosion of jobs caused by the digital revolution, proposing a tax on the wealth created by the use of robots to help underwrite the cost.
"We need a tax system that is based not on the number of workers in our companies but on the wealth created by the company," Hamon has argued.
Former prime minister Manuel Valls, who had been the frontrunner in the Socialist nominating contest but was beaten into second place on Sunday, has dismissed Hamon's proposal as unrealistic.
"I want nothing of these mirages that evaporate in an instant and that sow disillusionment (and) bitterness," Valls told a rally.
But many voters appear to have warmed to Hamon.
"Unlike the others he seemed sincere, natural and clear" during the debates, said Francois Moren, a 57-year-old travel agent, after voting for him on Sunday.
Hamon, an admirer of US Senator Bernie Sanders, had what he called an "inspiring and stimulating meeting" with the losing Democratic presidential candidate last September.
Hamon, who resigned as education minister in protest at what he saw as the Socialist government's rightward drift, had kept a low profile for two years before throwing his hat in the ring.
He attacked Valls' government, saying he could not support policies that neither reduced unemployment nor stimulated growth.
The government had ejected Hamon in 2014 along with then economy minister Arnaud Montebourg.
He scored third in Sunday's first-round primary vote and immediately threw his weight behind Hamon.
Hamon, who began his political life as a student activist in the 1980s, has pledged to tackle rampant inequality in the French school system and wants to legalise cannabis.
Hailing from western Brittany, the blue-eyed, fast-talking Hamon is the son of a secretary mother and a dockworker father who moved the family to Senegal for several years while Hamon was a child.
Hamon and Montebourg, who were among the founders of the dissident New Socialist Party in 2003, have both portrayed themselves as an antidote to Valls' right-leaning image.
The father of two with his partner Gabrielle Guallar, Hamon has a degree in history.
In 1986, aged 18, he joined massive student protests against proposed reforms that would have raised tuition fees, allowed universities to admit students selectively and abolished state diplomas, which give graduates equal qualifications regardless of where they study.
Then rightwing president Jacques Chirac had to withdraw the proposal in the face of the protests in which one person was killed and some 200 injured.
Hamon's role in the student movement led to his becoming president of the Movement of Young Socialists from 1993 to 1995.
He went on to work in the cabinet of Martine Aubry, the left-leaning social affairs minister from 1997 to 2000, becoming her spokesman when she was named first secretary of the Socialist Party in 2008.