women and two men were killed on Sunday along with their Nigerien guide and driver in a wildlife haven about an hour’s drive southeast from the capital Niamey.
The victims worked for French NGO Acted and were aged between 25 and 30.
The national tribute in the VIP section of Paris’s Orly airport will be closed to the media. Castex will be joined by several senior ministers.
“It’s important that the nation pays homage to them,” said Jerome Bertin, the head of France Victimes federation.
“Their families want their commitment to be really cited… they were not tourists killed in Niger but young people engaged in helping the people of this country.”
The country, one of the poorest in the world, is struggling with incursions by Islamists from both Nigeria to the south and Mali to the west.
In Paris, French anti-terror prosecutors said they would investigate charges of “murders with links to a terrorist enterprise” and “criminal terrorist association”.
There have been no claims of responsibility so far.
A judicial source in Paris told AFP the attack was “premeditated” and “targeting Westerners”.
French President Emmanuel Macron described it as “manifestly a terrorist attack” and said there would be repercussions.
“We’re pursuing action to eradicate the terrorist groups, with the strengthened support of our partners,” Macron said.
The president did not elaborate on the exact nature of the measures envisaged but Castex said the “odious crime” would not go unpunished.
Acted has decided temporarily to suspend work in Niger but has stressed it will not pull out of the country.