"Each asylum demand must be examined rapidly," he told members of the ruling Socialist Party gathered in the western town of La Rochelle.
Migrants and refugees who "are fleeing war, persecution, torture, oppression, must be welcomed," he said, adding that the rule should be to treat them with "dignity."
Earlier on Sunday Socialist party activists observed a minute of silence in honour of the over 2,000 migrants killed since January in shipwrecks on the Mediterranean.
Valls also quoted a line from a plaque hanging inside the Statue of Liberty in New York which reads: "Give me your tired, your poor, your huddled masses yearning to breathe free, the wretched refuse of your teeming shore. Send these, the homeless, tempest-tost to me, I lift my lamp beside the golden door!"
Valls however underscored that illegal migrants coming over for economic reasons would be dealt with "firmly."
"Our task is to find lasting responses founded on the values of humanity, responsibility and firmness," he said.
Earlier in the day, Foreign Minister Laurent Fabius told Europe 1 radio that the attitude of a number of eastern European countries towards the migrant crisis facing the EU is “scandalous".
Referring to an anti-migrant barrier in Hungary, Fabius said, "When I see a certain number of European countries, particularly in the east, who do not accept quotas (of migrants), I find it scandalous.”
Germany has acted "courageously," with the support of France, "but Europe as a whole must assume its responsibilities," said Fabius.
He specifically pointed to a razor-wire barrier along Hungary's border with non-EU member Serbia, aimed at keeping out migrants, which Fabius said "did not respect Europe's common values".