En Marche will announce its candidates for the legislative elections on Thursday.
Benoit Hamon, the unsuccessful Socialist Party candidate in the presidential contest, said he would set up a new movement after several of his hallmark proposals during that campaign were abandoned by his own party.
Mr Valls is planning to run in the Essonne department, his fiefdom south of Paris, but Republic on the Move officials said his nomination will not be automatic. "(Vall's) voice is not insignificant, but his candidacy will be treated like anyone else's".
Although every recent French presidential election has been followed by the winner's party going on to take control of parliament, the outcome this time around has been made murky by Macron's lack of an established base.
Hamon came in a distant fifth in the first round of France's presidential election, capturing just over 6 percent of the vote, the Socialist Party's worst result since 1969.
It remains to be seen whether Macron's newly renamed "Republique en Marche!"
"If some (people) want to leave and go apart, they can do so and let us work", he said.
But neither party got through to Sunday's presidential runoff, when Macron defeated the far-right's Marine Le Pen.
"The old parties are dying or are already dead", Valls said.
Valls's move could be a boost for Macron, who needs to secure a majority in the elections to have a realistic chance of implementing his ambitious reform plans.
"As of today, he (Valls) does not fit the criteria that would allow the investiture committee to take him on, so., (until he joins) the national investiture committee that I preside over can not consider the candidature of Mr. Valls", Jean-Paul Delevoye, who chooses Macron's party's candidates, told Europe 1 radio on Wednesday.
The same source referred to an opportunity of a bilateral meeting between Cyprus President Nicos Anastasiades and President Macron on the sidelines of the next EU Summit, as well as to Macron's expected participation in the meeting of the seven EU southern countries heads of state or government - France, Italy, Spain, Portugal, Greece, Cyprus and Malta - which will take place in Cyprus in the Autumn.
Last December, Valls was replaced as prime minister by Bernard Cazeneuve, who in his previous role as interior minister had overseen France's reaction to jihadist attacks.
New members are flocking to French President-elect Emmanuel Macron's party as it prepares for the elections to the National Assembly next month - starting with a change in its name.