She was discovered carrying a 10-month-old son among a group that had escaped from a Boko Haram hideout in Sambisa Forest, the army said.
"This morning as part of the operation we've been conducting, we rescued one of the Chibok girls. ... Our troops in Pulka rescued her along with a Boko Haram member," Maj. Gen. Lucky Irabor said at a press briefing Saturday.
The girl, identified as Maryam Ali Maiyanga, is said to have been among the 276 girls and women that Boko Haram militants herded from bed in the middle of the night at a boarding school in Chibok in April 2014.
As many as 57 girls escaped almost immediately.
21 Chibok girls released last month
Saturday's announcement comes nearly a month after 21 Chibok girls were freed in a deal brokered by the International Committee of the Red Cross and the Swiss government.
That release followed an apparent split in the terror group, with ISIS introducing a new leader
, according to a Boko Haram insider. Boko Haram has long had links with ISIS, pledging allegiance to the Islamic militant group in March 2015.
Roughly 196 girls are still unaccounted for.
The Nigerian government last month indicated it was negotiating with the terror group for the release of about 83 more girls. The rest -- about 113 -- have either died, been married off or become radicalized
and don't want to leave their Boko Haram kidnappers, sources told CNN.
The kidnappings sparked global outrage, with high-profile figures such as first lady Michelle Obama and Nobel Peace Prize winner Malala Yousafzai lending their weight to the #BringBackOurGirls campaign.
The campaign sent out a tweet, cheering the news about the discovery of the girl.
Chibok is in northeastern Borno state, where Boko Haram is strongest. The militants were believed to have taken their Chibok captives to the Sambisa Forest, a reputed stronghold.
Two years ago, when CNN first visited Chibok after the mass abduction, parents described how they had followed their daughters' trail to the edge of the Sambisa Forest. But they were unable to go further into the dense vegetation.
The terrorist group aims to impose a stricter enforcement of Sharia law across Africa's most populous nation, which has a Christian-majority south and a Muslim-majority north.
Since the group launched an uprising in the northeast in 2009, more than 20,000 people have been killed in the conflict and about 2 million people have been displaced, according to the UN Office for the Coordination of Humanitarian Affairs.