The girl was found by government troops while she was escaping, Femi Adesina said, without giving details.
She was among 276 girls seized by Boko Haram from north-eastern Chibok town in 2014, sparking global outrage.
A total of 103 of the girls have been released so far, including 82 earlier this month in a prisoner swap.
The 82 girls, who met Nigerian President Muhammadu Buhari on 7 May, are expected to be reunited with their families later this week.
They were escorted to a reception in the capital Abuja by armed soldiers, after a check-up at a medical centre.
"I cannot express in a few words how happy I am to welcome our dear girls back to freedom," Mr Buhari told the girls in Abuja, according to his office.
"On behalf of all Nigerians, I will like to share my joy with you."
The number of Boko Haram suspects released by the authorities in exchange for the girls remains unknown.
Last month, President Buhari said the government remained "in constant touch through negotiations, through local intelligence, to secure the release of the remaining girls and other abducted persons unharmed".
Aside from the Chibok girls, Boko Haram has kidnapped thousands of other people during its eight-year insurgency, which is aimed at creating an Islamic caliphate in north-eastern Nigeria.
The government says more than 30,000 people have been killed, and hundreds of thousands forced to flee their homes.