Assailants on motorbikes killed 37 people, including 14 children, in Tillaberi region, according to interior ministry.
Niger has declared 48 hours of national mourning after 37 people were killed in a fresh massacre by suspected rebels.
“Flags will be flown at half-mast across the country” with immediate effect, a government statement said on Wednesday.
It reaffirmed the government’s determination to “pursue the fight against terrorism until the final victory”, urging greater vigilance among the population.
On Monday, gunmen on motorbikes launched an attack on villagers in Darey-Daye as they were tending their fields.
Four women and 13 children were among the 37 people killed.
The massacre bore the hallmarks of rebel attacks that have battered western Niger since 2015, when an armed campaign spread from Mali.
Darey-Daye, which was hit in an attack in March that killed 66 people, lies in the Tillaberi region, which has borne the brunt of the bloodshed.
The village is located in the department of Banibangou, in the so-called “tri-border” area where the frontiers of Niger, Burkina Faso and Mali converge.
The area is notorious for attacks by groups linked to al-Qaeda and ISIL (ISIS).
According to a toll issued last week by Human Rights Watch, more than 420 civilians have been killed in rebel attacks in Tillaberi and the neighbouring region of Tahoua this year, and tens of thousands of people have fled their homes.