"I am Chelsea Manning. I am female," the U.S. Army private said in a statement read on the Today show.
"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."
Manning added: "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's lawyer, David Coombs, said on Today that Manning waited until now to announce his transition so as not to overshadow his criminal case.
Manning's gender identity disorder has been well documented. At one point in his Army career, he confided his confusion over his gender to a supervisor in an email and attached a photo of himself dressed as a woman.
Coombs said Thursday that he hopes Fort Leavenworth will "do the right thing" and provide Manning with hormone therapy in prison, adding that he will fight for that right if his client is denied it.
Coombs said he did not know whether Manning also planned to seek sex reassignment surgery.
The ultimate goal of the gender change is not to be grouped with female inmates, Coombs added, but rather for Manning "to be comfortable in her own skin."