Mannning, 25, was sentenced on Wednesday for his part in one of the biggest data breaches in US history.
A statement attributed to him was read out on NBC News’s Today program on Thursday.
It said: “I am Chelsea Manning. I am female. Given the way that I feel, and have felt since childhood, I want to begin hormone therapy as soon as possible. I hope that you will support me in this transition.”
“I also request that, starting today, you refer to me by my new name and use the feminine pronoun (except in official mail to the confinement facility). I look forward to receiving letters from supporters and having the opportunity to write back.”
Manning gave more than 700,000 military and diplomatic papers, and battlefield footage, to Julian Assange‘s anti-secrecy website, and was found guilty by a military court of 20 charges ranging from espionage to theft.
The 35-year sentence is the longest jail term ever handed out in the US for leaking information to the media, but Manning’s lawyers say he could be out in seven with good behaviour.
Supporters, who see Manning as a whistleblower rather than a traitor, have called on US President Barack Obama to pardon the half-Welsh army private.
In the absence of a pardon, Manning is expected to serve his sentence at Fort Leavenworth military prison. There are no facilities for hormone therapy or gender-reassignment surgery, according to a spokesman.
In Manning’s trial, his lawyers said his gender identity issues had influenced his behaviour.
The court saw an email written in 2010 in which Manning said he was transgender and had joined the army to “get rid of” the impulse.
The email, entitled My Problem, included a photo of Manning wearing a blonde wig and lipstick.