Manning - a former army analyst - was recently charged with aiding the enemy and stands accused of downloading thousands of classified documents that ultimately ended up on WikiLeaks.
"It's virtually impossible to distinguish what WikiLeaks did and what The New York Times [which published the papers] did at the time. I identify very much with Bradley Manning," Ellsberg told students during a recent lecture at Princeton University.
"I don't think anyone should be prosecuted ... for releasing information to Congress or the public that reveals criminal behavior."
Ellsberg also defended WikiLeaks founder Julian Assange for publishing the controversial material.
"[Although Assange] didn't redact enough in my opinion, I'm confident that he does not have intent to harm the United States or aid the enemy."
According to Ellsberg, the Bush administration had actually caused more harm in a post 9/11 world than either Manning or WikiLeaks - by engaging in warrantless domestic wiretapping operations and invading Iraq.
"Here is a case where Cheney, Bush and Rumsfeld were as worthy of going before the International Court of Justice - for the crime of aggression, not genocide - as any of the defendants at Tokyo or Nuremberg," Ellsberg claimed.
The former military analyst concluded his talk by reiterating that Bradley Manning had done "the right thing."
"He assumed judgment... He is a hero. [Yet], the U.S. government will never see him other than as a rat, a snitch, a traitor, a bad guy... He's been successfully defamed," added Ellsberg.
Manning is currently being held in a medium-security facility at Fort Leavenworth, Kansas, where he is awaiting a military trial. A total of 22 charges have been leveled against the soldier including: aiding the enemy; wrongfully causing intelligence to be published on the Internet; theft of public property or records; transmitting defense information; and fraud and related activity in connection with computers.
If found guilty, Manning will likely spend the rest of his life in prison without the chance of parole.