Miller, who spent 12 weeks in a prison near Washington, was set free after her source waived her pledge of confidentiality, The Times said on Thursday.
"That source was Lewis Libby, Vice-President Dick Cheney's chief of staff," the daily said, citing "people who have been officially briefed on the case".
She will testify as soon as Friday, the statement said.
Miller speaks out
"I went to jail to preserve the time-honoured principle that a journalist must respect a promise not to reveal the identity of a confidential source. I chose to take the consequences - 85 days in prison - rather than violate that promise," she said.
"I am leaving jail today because my source has now voluntarily and personally released me from my promise of confidentiality regarding our conversations relating to the Wilson-Plame matter.
"This enables me to appear before the Grand Jury tomorrow. I'll say nothing more until after my testimony."
Miller was sent to prison 6 July after a showdown between the US government and the press over a case involving the White House, press freedom and the rationale for the Iraq war.
She had refused to name her sources to a federal prosecutor examining which official leaked the name of CIA spy Valerie Plame.
Plame's husband, former US ambassador Joseph Wilson, claimed her cover was blown in revenge for an article he penned in The New York Times, criticising President George Bush's justification for war with Iraq.
Another journalist, Time Magazine's Matthew Cooper, almost met the same fate but was granted a last minute reprieve after his source cleared him to testify.
Both reporters were sentenced to 18 months in jail for contempt of court.