"I'm not going to take any risks," Valls told BFM TV. "I will vote for Emmanuel Macron."
French opinion polls show Macron winning the election in a second-round vote where he would face off against Le Pen with Socialist Benoit Hamon set for a humiliating fifth place in the first round eliminator.
Valls said his choice did not mean he would campaign behind the 39 year-old Macron - a fellow minister in President Francois Hollande's government from 2014, but who quit last year to prepare a presidential bid under his own political banner En Marche! (Onwards!).
"I have nothing to negotiate and am not asking for anything, I'm not joining his camp," Valls said.
Valls' endorsement is a mixed blessing for Macron, who has sought to avoid being cast as the candidate of the unpopular outgoing Socialist administration, instead pitching himself as willing to bridge the traditional left-right political divide.
"I'm suspicious of the hidden agenda of politicians," Macron told a news conference on Tuesday amid growing talk that Valls was about to make his intentions known.
Valls, who lost to radical leftwinger Benoit Hamon in the primaries, is seen as waiting in the wings to pick up the pieces of the Socialist party after the presidential race and build a reformist force in parliament to get a say in a Macron parliamentary majority.
The pro-business former premier has been scathing about his former education minister.
Hamon signed a motion of no-confidence against Valls' government last year, when the ruling Socialists split over a controversial labor reform led by Valls.
On Wednesday, Hamon shot back.
"I'm not surprised," he told France 2 television. "This sort of soap opera is meant to weaken me. I'm running my campaign by talking about the French's daily life, not Valls' life."
/Reuters/
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