The former Army intelligence analyst disclosed the decision in a statement provided to NBC's "Today" show.
"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.
The statement asked people to use the feminine pronoun when referring to Manning. It was signed "Chelsea E. Manning" and included a handwritten signature.
The Associated Press Stylebook calls for use of the pronoun that is either an individual's preference or is consistent with the way the person lives publicly. The news agency said in a statement it would let that "be our guide as this story develops."
However, Leavenworth spokesman George Marcec said later Thursday that if Manning wants to go by Chelsea in prison, a name change would have to be approved in court and then a petition submitted with the Army to change its records.
The AP said it was seeking additional details from Manning's attorney, David Coombs, and until then would use only gender-neutral terms in reference to Manning.
Coombs did not respond to email and telephone messages but told "Today" he hopes Leavenworth officials will accommodate Manning's request for hormone treatment, which typically involves high doses of estrogen to promote breast development and other female characteristics.
However, George Wright, an Army spokesman at the Pentagon, said the Army does not provide such treatment or sex-reassignment surgery. He said soldiers behind bars are given access to psychiatrists and other mental health professionals.
A lawsuit could be in the offing. Coombs said he will do "everything in my power" to make sure Manning gets his way. And the American Civil Liberties Union, the Human Rights Campaign and other advocates for gays, bisexuals and transgender people said Manning deserves the treatment.
"In the United States, it is illegal to deny health care to prisoners. That is fairly settled law," said Mara Keisling, executive director of the National Center for Transgender Equality. "Now the Army can claim this isn't health care, but they have the weight of the medical profession and science against them."
A Federal Bureau of Prisons policy implemented last year requires federal prisons to develop treatment plans, including hormone treatment if necessary, for inmates diagnosed with gender-identity disorder. But the bureau oversees only civilian prisons.
Manning's case appeared to be the first time the therapy had come up for a military prisoner.
Manning, 25, was convicted of Espionage Act violations and other crimes for turning more than 700,000 classified military and diplomatic documents over to the secrets-spilling website WikiLeaks. Coombs said the soldier could be paroled from prison in as little as seven years.
After sentencing, Manning was returned Thursday to Leavenworth.
Leavenworth is an all-male prison. But the staff has some leeway to separate soldiers from the other inmates based on the risk to themselves and others, Marcec said.
Manning would not be allowed to wear a wig or bra, and would have to meet the military standard for hair, Marcec said.
Advocates said gays and transgender people are more susceptible to sexual assault and other violence in prison.
"She most likely will need to be placed with a female prison population because she identifies as female," said Jeffrey Parsons, a psychology professor at Hunter College in New York.
Under a special agreement, the Army sends its female prisoners to a Navy women's jail in Miramar, Calif. It also has an agreement under which it can send soldiers to federal civilian prisons.
Greg Rinckey, a former Army prosecutor and now a lawyer in Albany, N.Y., said Manning's statement could be a ploy to get transferred to a civilian prison.
"He might be angling to go there because he believes life at a federal prison could be easier than life at the disciplinary barracks at Fort Leavenworth," Rinckey said.
He also said the military is adamant about not providing hormone treatment: "You enlisted as a male, you're a male, you're going to be incarcerated as a male."
Associated Press writers Lauran Neergaard and Sagar Meghani in Washington and John Milburn in Topeka, Kan., contributed to this report.
5 questions about Manning's future
The Army private sentenced to 35 years in prison for leaking reams of classified information to WikiLeaks issued a statement Thursday as a transgender woman. The soldier asked to be called Chelsea Manning instead of Bradley Manning and stated a desire for hormone therapy. Here are some questions and answers about this change:
When did Manning come to this realization? • According to Manning's online chats with confidant-turned-informant Adrian Lamo, it was in the first few months of Manning's deployment to Iraq, which started in October 2009. Manning spent a few days cross-dressed while on leave in the United States in January-February 2010 and created Twitter and YouTube accounts under the name Breanna Manning. Manning revealed the female persona in a photograph emailed to an Army supervisor in April 2010, about a month before his arrest for the WikiLeaks disclosures. The Bradley Manning Support Network said in July 2012 that as far as it knew, Manning wanted to be called Brad or Bradley "until he's able to get to the next stage of his life."
What will prison conditions be like for Manning? • Fort Leavenworth, Kan., is an all-male prison but the staff has some leeway to separate soldiers from the other inmates based on the risk to themselves and others, prison spokesman George Marcec said. Advocates said gays and transgender people are more susceptible to sexual assault and other violence in prison.
How can Manning dress in prison? • Leavenworth is an all-male prison. Uniforms are brown, usually "heavily starched," and inmates can buy their own shoes, which is where their fashion individuality shows, Marcec said. He said Manning won't be allowed to wear a wig or bra. Manning's hair will have to be kept to military standards, Marcec said.
Can Manning get hormone therapy in military prison? • Army spokesman George Wright said the Army does not provide hormone therapy or sex-reassignment surgery. He said soldiers behind bars are given access to psychiatrists and other mental health professionals.
What does Manning's lawyer say? • Defense attorney David Coombs told NBC's "Today Show" he will do "everything in my power" to make sure Manning gets his way. Military legal experts say that if Fort Leavenworth officials don't provide the hormone treatment, Coombs may seek to have Manning moved to a civilian federal prison, where hormone therapy is offered to those with gender-identity disorder.