"France-Saudi Arabia: 10 billion euros in contracts," he wrote in a tweet.
The deals include contracts and letters of intent between the two countries whose economic and political bonds have been strengthening.
Among the agreements is a Saudi order for 30 patrol boats.
The deals also cover energy, health, food, satellites and infrastructure, according to the prime minister's office.
The announcement came during the third high-level visit by French officials to the world's biggest oil exporter this year.
France has been reinforcing links with the conservative Islamic kingdom -- the Arab world's largest economy -- despite persistent criticism from rights activists of the kingdom's record on civil liberties.
For Saudi Arabia, expanding ties with France are part of an effort to build alliances beyond its traditional defence partner the United States, to counter Riyadh's regional rival Iran.
Valls on Monday night opened a forum to promote commercial ties between firms from the two countries.
He is on a regional tour which earlier took him to Jordan and Egypt, along with Defence Minister Jean-Yves Le Drian and Foreign Minister Laurent Fabius.
French aid agency Action Against Hunger (ACF) on Monday said Valls's visit seemed to favour the negotiation of contracts, notably military ones "with a powerful economic ally", instead of looking for diplomatic solutions to the war in Yemen, from which civilians are suffering.
Valls was due later Tuesday to hold talks with King Salman.
Valls's office said the prime minister would request "a gesture of pardon, humanity and clemency" for a member of the minority Shiite community, Ali al-Nimr, who is facing the death penalty.
He was only 17 when arrested in February 2012 after taking part in pro-reform protests and his case has raised international concerns.