He plans to live as a woman named Chelsea, he said in a written statement provided to the US television network NBC.
Manning’s struggle with gender identity disorder – his sense that he is a woman trapped in a man’s body – was a key part of his defence.
Lawyers had presented evidence of Manning’s struggle with gender identity, including a photo of the soldier in a blonde wig and lipstick that he sent to a therapist.
Manning had faced up to 90 years in prison. Prosecutors had wanted at least a 60-year sentence, saying it would dissuade other soldiers from following in his footsteps as a whistleblower. The defence suggested no more than 25 years so that Manning could rebuild his life.
A military judge announced the 35-year sentence in Manning’s court-martial.
With good behaviour and credit for the more than three years he has been held, Manning could be out in about six-and-a-half years, according to his defence lawyer David Coombs. Manning’s rank was reduced, he was dishonourably discharged and he forfeited his pay.
In his statement, Manning asked 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.
Mr Coombs said he is hoping officials at the military prison will accommodate Manning’s request for hormone therapy. If not, “I’m going to do everything in my power to make sure they are forced to do so”, he added.
Meanwhile, the fight to free Manning has taken a new turn, with Mr Coombs and supporters saying they will ask the army for leniency – and the White House for a pardon, which is unlikely.
Even Manning’s supporters have switched. During the sentencing hearing, they wore T-shirts reading “Truth”. Hours later, they had changed into shirts saying: “President Obama: Pardon Bradley Manning.”
Manning faces the stiffest punishment ever handed out in the US for leaking information to the media. He has been called both a whistleblower and a traitor for giving more than 700,000 classified military and diplomatic documents, plus battlefield footage, to the anti-secrecy website WikiLeaks.
Mr Coombs said he will file a request early next week that President Obama pardons Manning or commutes his sentence to time served.
Mr Coombs read from a letter Manning will send to the president in which he said: “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, an army intelligence analyst, digitally copied and released Iraq and Afghanistan battlefield reports and State Department cables while working in 2010 in Iraq. The soldier also leaked video of a 2007 Apache helicopter attack in Baghdad that mistakenly killed at least nine people.
The government alleged Manning was a traitor. The soldier 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.
Mr Coombs also will work on a separate process in which he can seek leniency from the army’s local area commander, who under military law must review – and could reduce – Manning’s convictions and sentence.