The party has been based on the Rue de Solferino in the chic Seventh Arrondissement of the capital since 1981 when ex-leader Francois Mitterrand was elected the first Socialist president under the current constitution.
Thirty-six years later, the party is fighting for survival after its debacle in the presidential election in May and a humiliating score in parliamentary elections in June, when President Emmanuel Macron's centrist party swept the board.
The decision to sell was made Tuesday evening by party leaders, with press reports saying the building, at 3,000 square metres (32,000 square feet), could raise 40 to 70 million euros ($50-85 million).
“It's a difficult decision because this place is symbolic,” party treasurer Jean-Francois Debat told reporters.
“It's based on two considerations. Firstly it's obvious there are financial constraints. It's also a political choice to prepare for the future.”
In the presidential election in May, the party's candidate Benoit Hamon finished fifth with a score of just 6.36 percent after a campaign in which many party leaders defected to Macron's centrist political movement.
In the parliamentary elections a month later, the number of Socialist lawmakers fell by more than 250 to just 30, leaving the party facing major financial problems after five years in power under ex-president Francois Hollande.
French political parties rely on both private donations and public subsidies which are paid on the basis of the number of their elected representatives.
After the parliamentary debacle, Socialist party chief Jean-Christophe Cambadelis resigned, leaving the movement rudderless and unable to make an impact in the first few months of Macron's rule.
(François Mitterrand is greeted by supporters at Solferino after being elected president in 1981. Photo: AFP)
Some rivals and analysts see the party facing ruin — with the French left currently dominated by fiery radical Jean-Luc Melenchon — while others point to past defeats and see the Socialists again rising from the ashes.
“It makes me sad because it's part of our history,” said senior Socialist Stephane Le Foll, a close ally of Hollande, as he commented on the sale of the headquarters on Wednesday.
But it “only reinforces my desire to try to rebuild something which is ambitious,” he added.