Glucksmann, 46, told France Inter radio on Monday that France was living in a "historic moment" with the far right at the gates of power. He urged the centre left to wage an "existential battle" and "rid our hearts of the spirit of defeat", vowing to restore a "pride in social democracy".
The primary victory marks the first time since the Socialist party was founded in 1969 that it will not field its own presidential candidate. The party, which suffered crushing defeats in the last two presidential elections, instead backed Glucksmann, who has cooperated with it on joint electoral lists in the past.
Current polling places Glucksmann at between 9% and 14%, well behind frontrunner Le Pen and also trailing centre-right contender Edouard Philippe and radical leftist Jean-Luc Melenchon, whose party France Unbowed boycotted the primary. Glucksmann refuses to work with Melenchon, leaving the left divided.
Glucksmann is staunchly pro-Europe and pro-Ukraine, and strikes a more reformist, centrist tone than the Socialists. He criticised President Emmanuel Macron, who will stand down next spring, for having "wasted 10 years" on climate inaction. Glucksmann said the centre left needed to regain "a form of radicality that we lost" through environmental policy, and that his intellectual background would help "better explain the moment we're living in" rather than relying on slogans.
His partner, Lea Salame, a star presenter on French public television, has stepped down from her role while he runs for president.