The Court of Appeals for the Armed Forces, in a 3-2 decision, rejected arguments by the Center for Constitutional Rights.
"Today's decision flies in the face of decades of First Amendment rulings in the federal courts that hold that openness affects outcome -- that the accuracy of court proceedings depends on their being open," said Shayana Kadidal, the senior attorney who argued the case for the center.
"Bradley Manning's trial will now take place under conditions where journalists and the public will be unable as a practical matter to follow what is going on in the courtroom. That ensures that any verdict will be fundamentally unfair, and will generate needless appeals afterwards if he is convicted."
The plaintiffs in the case included the Nation magazine, the television show "Democracy Now!" and individual journalists. Julian Assange, the founder of WikiLeaks, the organization Manning allegedly leaked classified documents to, also is a plaintiff.
Manning, a private first class who served as an intelligence analyst in Iraq, has entered a partial guilty plea, admitting to leaking classified material. But the Army decided to proceed with his court-martial.