The gunmen arrived on motorcycles and opened fire, the governor of Tillabéri region, Tidjani Ibrahim, told the French news agency AFP.
More information to come on The Point.
French President Emmanuel Macron's officer confirmed the deaths of the French citizens.
A source close to the environmental services said the assault took place around 11:30am (10:30 GMT) six kilometres (four miles) east of the town of Koure, in an area where the last herd of the so-called West African giraffes is found and about an hour's drive from the capital, Niamey.
The French government has previously warned its citizens from traveling in multiple parts of Niger to avoid Boko Haram and affiliates of the Islamic State.
The French foreign ministry said "checks are underway" into the reports of the attack.
A previous colonial power in the area, France introduced a union of West African and European allies in June to combat Islamist militants in the Sahel area that consists of Niger.
The French nationals worked for an worldwide aid group, Niger's defence minister Issoufou Katambé told Reuters news agency. "We found a magazine emptied of its cartridges at the scene", the source explained, adding that the tourists' vehicle belonged to the French aid organization ACTED.
The motorcyclists are believed to have been waiting for the tourists.
However the Tillaberi region is in a hugely unstable location, near the borders of Mali and Burkina Faso.
In January, Nigerien authorities banned the use of motorcycles in an attempt to curb the movements of jihadis.