In N'Djamena on Saturday and Niger's capital Niamey on Sunday, Valls will hold talks with respective presidents Idriss Deby and Mahamadou Issoufou.
Accompanied by his defence minister Jean-Yves Le Drian, Valls will on Saturday also visit a French military base near N'Djamena airport to meet troops.
For the French army, N'Djamena is a strategic point in the fight against Boko Haram fighters operating just tens of kilometres away in northeast Nigeria.
This intervention was replaced several months ago by a wider counter-terrorism operation, codenamed Barkhane, in five countries along the southern rim of the Sahara -- Chad, Niger, Mali, Mauritania and Burkina Faso.
French President Francois Hollande made similar trips to Chad and Niger in July.
The Barkhane force -- named after a crescent-shaped sand dune in the desert -- currently includes 3,200 French troops, plus special forces personnel, according to a government source.