According to reports, the girls were picked up by military helicopter from Banki area of Borno state where Boko Haram militants dropped them off earlier today.
The Federal Government has recently expressed its desire to negotiate the release of the abducted schoolgirls.
The girls were last seen in a video posted by the jihadist group in August, featuring its new leader appointed by Islamic State, Abu Musab al-Barnawi.
A vigilante group found Amina Ali Nkeki in May with her suspected militant husband and her new baby.
Boko Haram has given conditions including the release of its detained fighters in exchange for the schoolgirls. It is yet to be known if there was any exchange of sort.
The release of the 21 girls comes as President Buhari began a three-day trip to Germany to discuss assistance for the rebuilding of the northeastern part of Nigeria ravaged by Boko Haram.