Private Bradley Manning, who leaked thousands of classified documents to WikiLeaks, has been convicted of espionage but not of aiding the enemy.
Bradley Manning, 25, has been found guilty of 20 charges in total, including theft and computer fraud.
He had admitted leaking the documents to anti-secrecy organization WikiLeaks but said he did so to spark a debate on US foreign policy.
The leak is considered the largest ever of secret US government files.
Bradley Manning faces a maximum sentence up to 136 years. His sentencing hearing is set to begin on Wednesday.
In addition to multiple espionage counts, he was also found guilty of five theft charges, two computer fraud charges and multiple military infractions.
Bradley Manning stood and faced Judge Colonel Denise Lind as she read the decision on Tuesday. She said she would release detailed written findings at a later date.
He appeared to not react during the verdict, but his defense lawyer, David Coombs, smiled faintly as the not guilty charge on aiding the enemy was read.
“We won the battle, now we need to go win the war,” his defense lawyer, David Coombs said of the sentencing phase.
“Today is a good day, but Bradley is by no means out of the fire.”
Being found guilty of aiding the enemy could have had serious implications for people leaking documents in the future.
“The government’s pursuit of the ‘aiding the enemy’ charge was a serious overreach of the law, not least because there was no credible evidence of Manning’s intent to harm the USA by releasing classified information to WikiLeaks,” Amnesty International said in a statement.
Among the items sent to WikiLeaks by Pte. Bradley Manning was graphic footage of an Apache helicopter attack in 2007 that killed a dozen people in the Iraqi capital Baghdad, including a Reuters photographer.
The documents also included 470,000 Iraq and Afghanistan battlefield reports and 250,000 secure state department cables between Washington and embassies around the world.
Bradley Manning, an intelligence analyst, was arrested in Iraq in May 2010. He spent weeks in a cell at Camp Arifjan, a US Army installation in Kuwait, before being transferred to the US.
During the court martial, prosecutors said Bradley Manning systematically harvested hundreds of thousands of classified documents in order to gain notoriety.
With his training as an intelligence analyst, Bradley Manning should have known the leaked documents would become available to al-Qaeda operatives, they argued.
The defense characterized him as a naive and young soldier who had become disillusioned during his time in Iraq.
His actions, David Coombs argued, were those of a whistle-blower.
In a lengthy statement during a pre-trial hearing in February, Bradley Manning said he had leaked the files in order to spark a public debate about US foreign policy and the military.
Bradley Manning’s supporters rallied outside the court in Fort Meade and said they are planning to march to the White House on Tuesday evening.