The aiding the enemy charge was the most serious of 21 counts. It carried a possible life sentence without parole.
The former intelligence analyst was convicted of six espionage counts, five theft charges, a computer fraud charge and other military infractions. He faces up to 136 years in prison.
Manning's sentencing hearing is set to begin Wednesday.
Manning is accused of giving hundreds of thousands of classified documents to WikiLeaks. Prosecutors argued that Manning is a glory-seeking traitor, who knew the material would be seen by terrorist group al Qaeda.
Legal experts said an aiding-the-enemy conviction would have set a precedent because Manning did not directly give the classified material to al Qaeda. Manning's supporters also worried that a conviction on the most serious charge would have a chilling effect on other leakers.
The verdict by judge Col. Denise Lind follows about two months of conflicting testimony and evidence. Manning, a 25-year-old native of Crescent, Okla., has admitted to sending more than 470,000 Iraq and Afghanistan battlefield reports, 250,000 State Department diplomatic cables and other material, including several battlefield video clips, to WikiLeaks while in Iraq in early 2010. WikiLeaks published most of the material online.
Manning said he sent the material to expose war crimes and deceitful diplomacy. His lawyers say Manning is just a naive whistleblower who never intended for the material to be seen by the enemy. Manning claims he selected material that wouldn't harm troops or national security.
Manning's charges included eight federal Espionage Act violations, five federal theft counts, and two federal Computer Fraud and Abuse Act violations, each punishable by up to 10 years; and five military counts of violating a lawful general regulation, punishable by up to two years each.
The Associated Press contributed to this report.