Activists told the BBC that Amina Ali Nkeki was found by a vigilante group on Tuesday in the huge Sambisa Forest, close to the border with Cameroon.
In all, 218 girls remain missing after their abduction from a secondary school in north-east Nigeria in April 2014.
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.
‘Found with baby’
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.”
Sources told the BBC she came from the town of Mbalala, south of Chibok, from where 25 of the kidnapped girls came. A neighbour in Mbalala told the BBC that Amina was found with a baby.
An uncle, Yakubu Nkeki, told Associated Press news agency that Amina was later reunited with her mother in Chibok. She was 17 when abducted and is now 19, he said.
Amina is expected to be moved soon to Maiduguri, the capital of Nigeria’s Borno state.
In recent days, Nigerian media reported that the army had launched a number of operations against Boko Haram in the Sambisa Forest.
A Nigerian army spokesman, Colonel Sani Usman Kuka Sheka, issued a statement saying that a Chibok schoolgirl was among a group of people rescued by Nigerian troops.
During the 2014 attack, the gunmen arrived in Chibok late at night, then raided the school dormitories and loaded 276 girls on to trucks.
Some managed to escape within hours of their kidnapping, mostly by jumping off the lorries and running off into the bushes.
In total, 219 girls remained missing before this latest news.
A video broadcast by CNN in April this year appeared to show some of the kidnapped schoolgirls alive.
Fifteen girls in black robes were pictured. They said they were being treated well but wanted to be with their families.
The video was allegedly shot on Christmas Day 2015 and some of the girls were identified by their parents.
The Chibok schoolgirls, many of whom are Christian, had previously not been seen since May 2014, when Boko Haram released a video of about 130 of them gathered together reciting the Koran.
The abduction led to the #BringBackOurGirls campaign, that was supported by US First Lady Michelle Obama and Pakistani activist Malala Yousafzai.