At least 58 people were killed in two separate attacks on sellers near the border with Mali and Burkina Faso
The Nigerian leader, through his spokesman, Garba Shehu, expressed his condolences to families of victims and all citizens of Niger.
He assured Nigeria will continue to stand by its neighbours in the fight against terrorism.
Buhari noted that the killings underscore the security challenges facing Niger Republic as they prepare to inaugurate new President, Mohamed Bazoum.
The President called for strengthened collaboration against terror in the sub-Saharan region.
Buhari condemns masscare in Niger Republic