The gunmen rode in on motorcycles and opened fire, the governor of Tillabéri region, Tidjani Ibrahim, told the French news agency AFP.
They were in the Koure region which attracts tourists who want to see the last herds of giraffe in West Africa.
It is not yet clear who is behind the attack.
The French government warns people against travelling to large parts of the former French colony outside the capital Niamey.
