The girl, identified as Amina Ali, was found in Kaya, a city in north-central Nigeria, according to Pogu Bitrus, a Chibok leader active in the Bring Back Our Girls campaign.
A vigilante group recognized Ali, who was found with a 4-month-old baby presumed to be her child and three other young children, Bitrus said. It was not immediately clear who the three other young children were, he added.
Ali was taken back to Chibok, where she identified a member of her family, saying, "This man is my uncle," Bitrus said, adding that she refused to talk beyond identifying the man.
Ali and suspected Boko Haram terrorist Mohammed Hayatu, who claimed to be Ali's husband, were taken to Maiduguri for additional medical attention and screening, according to the Nigerian army.
Ali has been taken to a nearby military installation in Nigeria, and a group from Chibok has been sent to her home village of Balala, just south of Chibok, to find out more information from her father, according to Bitrus.
Boko Haram Islamic extremists stormed and firebombed the Government Girls Secondary School at Chibok on April 14, 2014, and seized 276 girls who were preparing to take science exams. Dozens of the students escaped during or shortly after the attack, but 219 remained missing, according to The Associated Press.