Valls’ office issued a statement that he would make a declaration at 1730 GMT at the town hall in Evry, just south of Paris, but it gave no further details.
His expected announcement follows a primary ballot in which Francois Fillon, a 62-year-old former prime minister, secured a resounding win to become the presidential candidate of the centre-right Les Republicains party.
France’s presidential election takes place in two rounds next April and May.
Valls has long been seen as a presidential candidate, and his status as the Socialists’ likely choice for 2017 was cemented further last week after Francois Hollande’s shock announcement that he would not seek a second term.
A snap opinion poll, conducted on Thursday night after Hollande’s statement, showed that Socialist voters and French voters as a whole wanted to see Valls win the party ticket to run for president next year.
The Socialists face a tough battle over whether they should be more centrist or veer more to the left to try and regain the popularity they have lost since Hollande was elected in 2012.