The prime minister's office announced only that he would make a statement at 6:30 pm (1730 GMT) from his political base in Evry, south of Paris.
But his entourage confirmed that he would put an end to the speculation over whether he would run -- seen as highly likely following President Francois Hollande's decision on Thursday not to stand.
The French left, in search of a new leader after Hollande's exit, will hold a US-style primary in January to choose a presidential candidate.
The first round of the presidential election in France is due to be held on April 23 with a decisive run-off on May 7. Polls show the far-right Marine Le Pen could triumph in the first round but would likely lose to the rightwing candidate Francois Fillon in the run-off.
The divided left is in disarray with former economy minister Emmanuel Macron also throwing his hat into the ring as an independent.
If Valls does announce a run for the Elysee presidential office, he will have to resign as prime minister, sparking a government reshuffle.
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