Valls and Justice Minister Jean-Jacques Urvoas visited the Nîmes prison on Monday morning and Valls promised a "specific, concrete, precise and costed" plan to reduce prison overcrowding by the autumn.
Urvoas is due to present a report on individual cells for detainees, which he said has become a "security imperative", to the Senate in September.
Three inmates in 9.0m² cell
The ministers spent two hours at the prison, where conditions were judged "inhuman and degrading" by the Council of State a year ago.
One inmate, Francis R, appealed to the European Court of Human Rights last year, having been forced to share a 9.0m² two-bed cell with two other men for 14 months.
A study on enlarging it was started this month, the ministers said.
France's prison population stood at 68,685 in May, four lower than the April 2014 record, but its prisons in theory had room for only 58,683 inmates.
That has led to systematic overcrowding, with 1,648 prisoners sleeping on mattresses on the floor, a rise of 61.7 percent in a year.
"It's not enough to announce the construction of prisons, which we will do, but above all we must finance them and then say what we will build," Urvoas said.
Valls pointed out that several prisons have been built recently and others, such as Paris's famous Santé prison, renovated, while 1,100 jobs have been created in the penal system.
"But," he added, "there's still a lot left to do."
Politics and penal policy
The construction of more facilities alone will not solve France'e prison overcrowding crisis, the nation's controller of prisons, Adeline Hazan, said ahead of the visit."History has shown that the more places are created, the more are filled," she told France Inter radio.
She disagreed with the call from right-wing parties for the creation of 10,000-20,000 places for prisoners, saying that former justice minister Christiane Taubira's proposal of 6,000 more in 2014 was adequate.
Taubira's efforts at penal reform, which sought to relieve pressure on prisons with more non-custodial sentences, came under fire from the right for allegedly being soft on crime.
She and Valls also failed to see eye to eye, and she resigned in 2015.
The prime minister on Monday made a point of remarking that "the government has taken action, especially since Jean-Jacques Urvoas has been justice minister".
Recruiting ground for violent Islamism
Prisons controller Hazan on Monday called for prison to be "truly a last resort" and advocated more measures like suspended sentences and conditional liberation.
But non-custodial sentences are politically sensitive, even more so since it has been revealed that one of the two men who murdered a French priest in church last month was wearing an electronic tag, while awaiting trial for alleged membership of a terrorist organisation.
On the other hand, prisons have been identified as key recruiting grounds for jihadi groups.
Overcrowding leads to leads to radicalisation, Hazan said, as well as to violence among inmates and between inmates and wardens.
The suicide rate in French prisons is twice the European average, a 2013 Council of Europe report found.
France has 188 prisons, including Europe's largest, Fleury-Mérogis, near Paris, which has a capacity of 2,857 but is now housing 4,539.