"I'm sorry that my actions have hurt people and have hurt the United States," he told a military judge, Colonel Denise Lind, at a sentencing hearing at Fort Meade, north-east of Washington on Wednesday.
Manning, convicted last month of espionage for his massive leak of classified US battlefield reports and diplomatic cables, said he was ready to face punishment for his actions.
"I want to go forward," he said. "I understand I must pay the price."
The 25-year-old US Army private faces up to 90 years in prison for his offenses, which include espionage and computer fraud.
He was acquitted of a more serious charge that he deliberately was "aiding the enemy" through the leak, which could have landed him in jail for life without parole.
The dramatic statement in court marked the first time Manning had expressed regret over his unprecedented leak, the biggest in American history.
The former junior intelligence analyst has become a folk hero to his supporters, who see him as a whistleblower lifting the lid on America's foreign policy.
Nobel Peace Prize petition
More than 100 000 people have signed a petition calling for his nomination for a Nobel Peace Prize.
But the US government has painted him as a reckless traitor who put his fellow soldiers and country in danger when he handed over 700 000 secret documents to the anti-secrecy website WikiLeaks while deployed in Iraq.
Manning's defence team has argued the US Army private was a naive but well-intentioned young man who hoped to ignite a public debate over the conduct of the American diplomats and troops abroad.
The defence has suggested that Manning's superiors ignored repeated signs of his emotional distress and should never have allowed him to deploy to Iraq or retain his security clearance.
Earlier at the sentencing hearing, experts testified that Manning was plunged into a solitary anguish as he struggled over his sexual identity amid a "hostile" military environment.
"Being in the military and having a gender issue does not exactly go hand in hand," Captain Michael Worsley, a military clinical psychologist, told the court.
"At the time, the military was not exactly friendly towards the gay community," he said.
Worsley diagnosed Manning with a personality disorder and then a "gender identity disorder", and said the soldier would have faced an agonising plight in the macho world of the military.
'Severe emotional stress'
"The pressure would have been incredible in an almost openly hostile environment … a hyper masculine environment," the doctor told the court.
The US Army private "was concerned about his job, and how he was perceived," Worsley added.
"He was super-critical of himself. He was feeling he was never good enough."
Another witness for the defence, David Moulton, a psychiatrist and expert in military criminal cases, said Manning was facing "hyper stress" at the time.
"He was under severe emotional stress" when he began "considering living as a woman", he said.
Manning had no one to confide in during his deployment outside Baghdad and had been raised by alcoholic parents.
"He had very limited social support, even at home," he said.
Striving for "something great" in his life, Manning believed he was fulfilling a moral obligation with his leaks and was "trying to prevent a civil war".
"He was under the impression that the information he leaked was going to change the world," he said. "In his opinion, it would lead to a greater good." – AFP