The cases asked the court to revisit an issue that it last dealt with more than 30 years ago — whether reporters can be jailed or fined for refusing to identify their sources.
The justices' intervention had been sought by 34 states and many news groups, all arguing that confidentiality is important in news gathering.
"Important information will be lost to the public if journalists cannot reliably promise anonymity to sources," news organizations including The Associated Press told justices in court papers.
Time magazine's Matthew Cooper and The New York Times' Judith Miller, who filed the appeals, face up to 18 months in jail for refusing to reveal sources as part of an investigation into who divulged the name of CIA officer Valerie Plame.
The ruling effectively ends Cooper and Miller's appeal on free-speech grounds, although Time said in a statement it would seek a new hearing in federal court on other grounds.
Miller said journalists "simply cannot do their jobs without being able to commit to sources that they won't be identified."
The Newspaper Association of America called on Congress to enact shield legislation that would protect reporters from having to disclose confidential sources.
Plame's name was first made public in 2003 by columnist Robert Novak, who cited unidentified senior Bush administration officials for the information. The column appeared after Plame's husband, former Ambassador Joseph Wilson, wrote a newspaper opinion piece criticizing the Bush administration's claim that Iraq sought uranium in Niger.
Disclosure of an undercover intelligence officer's identity can be a federal crime and a government investigation is in its second year. No charges have been brought.
U.S. Attorney Patrick Fitzgerald of Chicago, the special counsel handling the probe, told justices that the only unfinished business is testimony from Cooper and Miller.
Cooper reported on Plame, while Miller gathered material for an article about the intelligence officer but never wrote a story.
A federal judge held the reporters in contempt last fall, and an appeals court rejected their argument that the First Amendment shielded them from revealing their sources in the federal criminal proceeding.