The man and his Nigerien colleagues were travelling near the border with Mali on Wednesday when armed men on motorbikes surrounded their convoy.
The four Nigeriens have been freed and no demands have been made by the kidnappers.
It is not clear who was behind the incident, but Islamist militants have repeatedly carried out attacks in the region.
The man, who worked for the German non-governmental organisation Help, had been travelling the Tillaberi region of western Niger.
He was kidnapped near the town of Ayorou, about 200km (124 miles) north-west of the capital, Niamey.
His driver told the AFP news agency that the attackers beat them up, burnt the vehicle and then left with the German.