"I want to condemn this initiative with the greatest strength," Valls said.
"For parliamentarians to go without warning to meet a butcher.... I think it was a moral failing."
A cross-party group of four French lawmakers made an unofficial trip to the Syrian capital Damascus on Wednesday and held talks with senior ministers.
Three of the MPs -- not including one from the ruling Socialist party -- met with the Syrian leader.
"We met Bashar al-Assad for a good hour. It went very well," Jacques Myard, an MP from the opposition UMP party, said.
He described the trip as "a personal mission to see what is going on, to hear, listen."
France cut diplomatic ties with Syria in 2012 and supports the moderate Syrian opposition, seeking the removal of Assad from power.
Myard refused to reveal the content of the talks. Syrian state television said they had discussed "the state of Syrian-French relations, as well as the developments in the Arab world and Europe, especially with regard to terrorism."
According to the Syrian Observatory for Human Rights, a Britain-based monitoring group, the civil war in Syria has left more than 200,000 people dead.