"I am okay. I'm glad to be alive. Thank you all for your love. I will get through this," she wrote on the social media site, adding the hashtag, #standwithchelsea
Although she does not have any Internet access in prison, Manning joined Twitter in April 2015
. She uses a voice phone to dictate her tweets to a communications firm, which posts them on her behalf.
Her legal representatives released a statement Monday, confirming that she had attempted to take her own life.
"After not connecting with Chelsea for over a week, we were relieved to speak with her this morning," the statement read.
"Last week, Chelsea made a decision to end her life. Her attempt to take her own life was unsuccessful. She knows that people have questions about how she is doing and she wants everyone to know that she remains under close observation by the prison and expects to remain on this status for the next several weeks.
"She is someone who has fought so hard for so many issues we care about and we are honored to fight for her freedom and medical care."
Her lawyers also accused the government of a "gross breach of confidentiality" in releasing details of her medical status.
Following her hospitalization a week ago, her lawyers and family said that they had been kept in the dark about her condition.
After her suicide attempt, Manning was taken to a local hospital near the U.S. Disciplinary Barracks at Fort Leavenworth, Kansas, where she is being held for violating the Espionage Act.
U.S. Army spokesman Colonel Patrick Seiber confirmed to CNN that Manning was taken to the hospital "during the early hours of July 5" before returning to the barracks. The spokesman said that officials "continue to monitor the inmate's condition."
Manning, 28, is currently serving a 35-year prison sentence for her role in leaking government documents to WikiLeaks.
In May, her attorneys formally appealed that conviction, arguing that "(n)o whistleblower in American history has been sentenced this harshly."
Manning, formerly known as Bradley Manning, was born male. In 2013, she asked to be recognized as a woman, and last year, the Army agreed to provide her with hormone therapy.