Manning announced the decision Thursday in a 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 received 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 of time served, Manning could be out in as little as seven years, his attorney David Coombs said.
Coombs told "Today" he hoped officials at the military prison in Fort Leavenworth, Kan., will accommodate Manning's request for hormone treatment, but the Army said it doesn't provide it or sex-reassignment surgery.
"All inmates are considered soldiers and are treated as such with access to mental health professionals, including a psychiatrist, psychologist, social workers and behavioral science noncommissioned officers," Army spokesman George Wright said.
Coombs said he would do "everything in my power to make sure" Manning received hormone treatment. He did not respond to phone and emails from The Associated Press.
Manning's struggle with gender identity disorder - the sense of being a woman trapped in a man's body - was key at his court-martial.
Retired Sgt. 1st Class Paul Adkins testified as a defense witness, saying in April 2010, just a month before Manning was arrested, the soldier emailed him a picture of himself in a blonde wig and lipstick with a letter titled, "My problem."
"I don't know what to do anymore, and the only 'help' that seems to be available is severe punishment and/or getting rid of me," the email said. "All I do know, is that fear of getting caught has caused me to go to great lengths to consciously hide the problem."
Manning's attorney said the email was evidence the military knew of Manning's struggles, yet allowed him to stay in Iraq as an intelligence analyst and keep his security clearance.
Meanwhile, Coombs and supporters said they will ask the Army to reduce Manning's sentence and they want President Obama to grant a pardon.
"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 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."
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 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.
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."
Associated Press writer Eric Tucker in Washington contributed to this report.