In all, 218 girls remain missing after their abduction from a secondary school in north-east Nigeria in April 2014.
Activists told the BBC that Amina Ali Nkeki was found by a vigilante group on Tuesday in the Sambisa Forest, close to the border with Cameroon.
The girls were taken by militants from the Boko Haram Islamist group.
Amina was reportedly identified by a civilian fighter who recognised her. The fighter belonged to the Civilian Joint Task Force (JTF), a vigilante group set up to help fight Boko Haram.
Hosea Abana Tsambido, the chairman of the Chibok community in the capital, Abuja, told BBC Focus on Africa that Amina was found by the vigilantes after venturing into the forest to search for firewood.
“She was saying… all the Chibok girls are still there in the Sambisa except six of them that have already died.”