The reporter, Judith Miller, is to stay in prison until she agrees to give the name of her source or until the October end of the mandate of the grand jury that is investigating the case.
Another reporter, Matthew Cooper of "Time" magazine, avoided going to jail when he told the judge he had received permission from his source to reveal the source's identity.
Cooper told reporters the case marked a "sad day" for the United States.
"This is a sad day not only for journalists, but also for the country," Cooper said. "It is a sad time when two journalists, who are doing their job and are trying to keep confidentiality and reporting important stories, face the prospect of going to prison by keeping their confidences."
The grand-jury investigation is seeking to determine who in the Bush administration leaked to reporters the name of covert CIA operative Valerie Plame in 2003, and whether this violated U.S. law.
Plame's husband, a former U.S. diplomat, had publicly criticized the Bush administration's handling of the Iraq war before her name was leaked.
(Reuters/AP/AFP)For RFE/RL's full coverage of developments in Iraq, see "The New Iraq"