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.
Militant groups including Boko Haram operate in the area and violence by groups linked to al Qaeda and Islamic State has been on the rise in the Sahel region.
Despite this, tourists still visit to see the Niger giraffes, a sub-species distinguished by its lighter colour who settled in the area around 20 years ago and have been largely protected from poachers.