Manning, 29, was released from the Disciplinary Barracks at Fort Leavenworth, Kansas.
"First steps of freedom!!" Manning wrote alongside a photograph of sneaker-clad feet that she published on social media.
Manning was convicted by court- martial in 2013 of espionage and other offenses for furnishing more than 700,000 documents, videos, diplomatic cables and battlefield accounts to WikiLeaks, which publishes such information from anonymous sources, while she was an intelligence analyst in Iraq.
Once known as Private First Class Bradley Manning, she is likely to become a high-profile proponent for the transgender community, said Chase Strangio, an American Civil Liberties Union lawyer who has represented her.
Manning announced her gender transition while the US Army was keeping her in the men's prison and forcing her to wear a male haircut. She twice tried to commit suicide and faced solitary confinement as well as denial of proper health care, Strangio said.
Last year, the Defense Department lifted a ban against transgender men and women serving openly in the military.