Manning announced the decision in a written statement provided to NBC's "Today" show, asking supporters to refer to him by his new name and the feminine pronoun. The statement was signed "Chelsea E. Manning."
"As I transition into this next phase of my life, I want everyone to know the real me. I am Chelsea Manning. I am a female. Given the way that I feel, and have felt since childhood, I want to begin hormone therapy as soon as possible," the statement read.
Manning's defense attorney David Coombs told "Today" in an interview that he is hoping officials at the military prison in Fort Leavenworth, Kan., will accommodate Manning's request for hormone therapy.
"If Fort Leavenworth does not, then I'm going to do everything in my power to make sure they are forced to do so," Coombs said.
Coombs did not respond to phone and email messages from The Associated Press on Thursday.
Manning's struggle with gender identity disorder - the sense of being a woman trapped in a man's body - was key to the defense.
Attorneys had presented evidence of Manning's struggle with gender identity, including a photo of the soldier in a blond wig and lipstick sent to a therapist.
Meanwhile, the fight to free Manning has taken a new turn, with Coombs and supporters saying they will ask the Army for leniency - and the White House for a pardon.
Even Manning's supporters have pivoted. During the sentencing hearing Wednesday, they wore T-shirts reading, "truth," as they had for the entire court-martial. Hours later, they had changed into shirts saying, "President Obama: Pardon Bradley Manning."
"The time to end Brad's suffering is now," Coombs told a news conference after Manning's sentence was handed down. "The time for our president to focus on protecting whistleblowers instead of punishing them is now."
The sentence was the stiffest punishment ever handed out in the U.S. for leaking information to the media. With good behavior and credit for the more than three years he has been held, Manning could be out in as little as seven years, Coombs said. Still, the lawyer decried the government's pursuit of Manning for what the soldier said was only an effort to expose wrongdoing and prompt debate of government policies among the American public.
The sentencing fired up the long-running debate over whether Manning was a whistleblower or a traitor for giving more than 700,000 classified military and diplomatic documents, plus battlefield footage, to the anti-secrecy website WikiLeaks. By volume alone, it was the biggest leak of classified material in U.S. history, bigger even than the Pentagon Papers a generation ago.
Manning was to return to the military prison at Fort Leavenworth, Coombs said, adding that he didn't know precisely when the soldier would leave Maryland. Coombs said he will file a request early next week that Obama pardon Manning or commute his sentence to time served.
Coombs read from a letter Manning will send to the president that read: "I regret if my actions hurt anyone or harmed the United States. It was never my intent to hurt anyone."
Manning said the disclosure was done "out of a love for my country and a sense of duty to others."
The White House said the request would be considered "like any other application." However, a pardon seems unlikely. Manning's case was part of an unprecedented string of prosecutions brought by the U.S. government in a crackdown on security breaches. The Obama administration has charged seven people with leaking to the media; only three people were prosecuted under all previous presidents combined.
Coombs also will work in coming weeks on a separate process in which he can seek leniency from the local area commander, who under military law must review - and could reduce - Manning's convictions and sentence.
Manning, an Army intelligence analyst from Crescent, Okla., digitally copied and released Iraq and Afghanistan battlefield reports and State Department cables while working in 2010 in Iraq. Manning also leaked video of a 2007 Apache helicopter attack in Baghdad that mistakenly killed at least nine people, including a Reuters photographer.
Manning said the motive was exposing the U.S. military's "bloodlust" and generate debate over the wars and U.S. policy. The government alleged Manning was a traitor who betrayed his oath as a soldier in order to gain notoriety.
Manning was found guilty last month of 20 crimes, including six violations of the Espionage Act, but was acquitted of the most serious charge, aiding the enemy, which carried a potential sentence of life in prison without parole.
Whistleblower advocates said the punishment was unprecedented in its severity. Steven Aftergood of the Federation of American Scientists said "no other leak case comes close."
Daniel Ellsberg, the former defense analyst who leaked the Pentagon Papers in 1971, on Wednesday called Manning "one more casualty of a horrible, wrongful war that he tried to shorten." Ellsberg also was charged under the Espionage Act, but the case was thrown out because of government misconduct, including a White House-sanctioned break-in at the office of Ellsberg's psychiatrist.
Others disagreed.
Gabriel Schoenfeld, a senior fellow at the conservative Hudson Institute think tank and author of the book "Necessary Secrets," welcomed Manning's punishment.
"The sentence is a tragedy for Bradley Manning, but it is one he brought upon himself," he said. "It will certainly serve to bolster deterrence against other potential leakers."
But he also warned that the sentence will ensure that Edward Snowden - the National Security Agency leaker who was charged with espionage in a potentially more explosive case while Manning's court-martial was underway - "will do his best never to return to the United States and face a trial and stiff sentence."
Coombs said that he was in tears after the sentencing and that Manning comforted him by saying: "Don't worry about it. It's all right. I know you did your best. ... I'm going to be OK. I'm going to get through this."
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Associated Press writer Eric Tucker in Washington contributed to this report.
