Chelsea Manning, 28, is currently serving a 35-year sentence. She tried to commit suicide in a cell at Fort Leavenworth Prison in Kansas, according to CNN.
One source told TMZ that she tried to hang herself.
Manning was rushed to a nearby hospital for treatment before being returned to prison.
U.S. Army spokesman Colonel Patrick Seiber said that Manning was found in her cell "during the early hours of July 5th" and added that officials "continue to monitor the inmate's condition."
The transgender soldier, formerly known as Bradley Manning, was convicted in 2013 of espionage and other offenses for sending WikiLeaks more than 750,000 digital files including videos of American-led airstrikes in Baghdad, Iraq, Afghanistan war logs and Guantanamo Bay files.
Manning was busted after she confided in a friend, Adrian Lamo, who reported her to Army Intelligence.
She will be eligible for parole after serving eight years. At sentencing, she revealed that she had felt like a female since childhood and wanted to be known as Chelsea.
Last year, the Army agreed to provide her with hormone therapy. Despite her ongoing transition, she is being kept in an all-male facility.
In May, Manning's attorneys appealed the conviction, arguing that "(n)o whistleblower in American history has been sentenced this harshly."
Manning recently wrote a column from prison for The Guardian about the new U.S. rules allowing transgender people to openly serve in the military.
"Gender presentation should reflect the person that you are," Manning wrote. "When you lose control of your gender presentation you lose an important aspect of your identity and existence. By setting so many caveats, time lines, standards, and training, the military is making this far, far, more complicated and bureaucratic than it needs to be. The simple reality is that we are who we say we are."