Valls said he wanted to do all he could to ensure that far-right leader Marine Le Pen, second-placed in opinion polls, did not win power.
"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 presidency in a second-round vote on May 7 where he would face off against Le Pen. They show Socialist Benoit Hamon set for a humiliating fifth place in the first round eliminator on April 23.
Valls said his choice did not mean he would campaign for 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! french term for Onwards!.
Valls, who lost to radical left-winger Hamon in the Socialist primaries, is seen by political sources and experts as likely to wait in the wings and seek to build a reformist parliamentary force that would be distinct from En Marche!, but which could get a say in its parliamentary majority should Macron become president.
"I have nothing to negotiate and am not asking for anything, I'm not joining his camp," Valls said.
Macron was also quick to say that while he was grateful for the support, he did not plan to bring Valls into his government.
"I shall be the guarantor of new faces, new ways of doing things," he said on Europe 1 Radio.
The news came a day after the campaign of third-placed candidate Francois Fillon suffered a further blow when his wife Penelope was put under formal investigation for misuse of public funds over allegations she did little work for a tax-funded salary as his parliamentary assistant.
Penelope Fillon, 61, made no comment on Tuesday. She said in a newspaper interview this month that the work she had carried out for her husband was real. Fillon himself, put under investigation last week, has repeatedly denied wrongdoing.
Valls' endorsement is a mixed blessing for Macron, even though their political views are not far apart.
The ex-investment banker, who has never run for elected office before, has sought to avoid being cast as the candidate of the unpopular outgoing Socialist administration and pitched himself instead as a bridger of the left-right political divide.
For the Socialists, Valls' decision comes from a man who represents Hollande's rightward turn in the middle of his five-year mandate towards the business-friendly reforms that upset the left and alienated core voters.
It highlights the already obvious left-right split it is suffering.
The rancour in the party came across in the reaction from Arnaud Montebourg, a leftist who was Macron's predecessor as economy minister under Hollande.
"Everybody now knows the value of a commitment signed on the honour of a man like Manuel Valls," he said on Twitter. "It's the value of a man who has no honour."
Hamon signed a motion of no-confidence against Valls' government last year, when the ruling Socialists split over a labour reform led by Valls.
"I'm not surprised," Hamon told France 2 television on Wednesday. "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."
Valls has personal experience of the Socialists election shock 2002 when Le Pen's father, National Front founder Jean-Marie Le Pen, beat Socialist contender Lionel Jospin to the presidential runoff against conservative Jacques Chirac who went on to be president.
Valls, in his political youth at that time, was Jospin's spokesman.
(Reporting by Sudip Kar-Gupta and Michel Rose, Editing by Brian Love, Andrew Callus and Alison Williams)
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)