Niger has not yet commented on the incident.
Air Info reported that the kidnappers showed up at Ms Gretzmacher’s house with guns and forced their way inside. They reportedly took nothing else.
Ms Gretzmacher is said to have led projects in education, health, women’s empowerment and culture.
Austria’s foreign ministry said it was working with the EU delegation and authorities on the ground.
Niger has been battling an Islamist insurgency for years.
The military junta is under pressure for its failure to stop militant attacks, which is one of the justifications for ousting democratically elected President Mohamed Bazum in July 2023.
Niger, along with its neighbors Mali and Burkina Faso, is fighting attacks by groups linked to al-Qaeda and ISIS.
After the coup, Niger called on France and other Western powers to withdraw their military bases and formed a security alliance with the neighboring junta of Mali and Burkina Faso.
