Manning gave an unsworn statement Wednesday, which means he cannot be cross-examined by prosecutors. He began with an apology.
He says "I'm sorry that my actions hurt people. I'm sorry that it hurt the United States."
Manning says he understood what he was doing and the decisions he made. However, he says he did not believe at the time that leaking the information would cause harm.
Manning was earlier convicted of espionage, theft and computer fraud charges for leaking classified information to WikiLeaks while working as an intelligence analyst in Iraq in 2010. He faces up to 90 years in prison.
Wednesday, an Army psychologist testified that Manning's private struggle with gender identity in what he called a hostile workplace put pressure on the soldier. Manning eventually came out to the therapist.
Manning's lawyers contend that Manning showed clear signs of deteriorating mental health that should have prevented commanders from sending him to a warzone to handle classified information.
In addition to Manning, his sister and aunt are also on the defense witness list.
The Associated Press contributed to this report.