She is first among 276 girls kidnapped by Boko Haram from their secondary school in north-east Nigeria in April 2014.
She has been found by a vigilante group on Tuesday in the Sambisa Forest, close to the border with Cameroon, reported the BBC citing an activist.
A civilian fighter has reportedly recognised her.
Launched in 2002, Boko Haram - which in the local Hausa language means "Western education is forbidden", a reference to the initial focus of its Islamist agenda - became a force to be reckoned with in 2009 when it raided police stations and government buildings in Maiduguri, and other northern cities.
US State Department added Boko Haram in terrorist organisation in November 2013.