Rescue worker Almoustapha Alhacen said the corpses were in a severe state of decomposition and had been partly eaten, probably by jackals.
Those found are thought to be migrant workers and their families. Most were women and children.
Niger lies on a major migrant route between sub-Saharan Africa and Europe.
According to Mr Alhacen, one of the vehicles that the migrants were travelling in broke down some time after they left Arlit at the end of September or beginning of October.
“The corpses were decomposed; it was horrible,” he said.
“We found bodies across a wide area. We had no idea what to expect because we didn’t know how many people had been in the vehicles.”