Kennedy issued his order in response to lawsuits by civil liberties groups in 2005 after news reports disclosed the wiretapping.
The department had argued that the memos were protected attorney-client communications and contain classified information.
But Kennedy said that the attorney-client argument was "too vague" and that he would have to look at the documents himself to determine if that argument is valid and also to see if there is information that can be released without endangering national security.
Justice Department spokesman Dean Boyd said Saturday the department is reviewing the opinion and will "respond appropriately in court."
Shortly after the Sept. 11 attacks, Bush authorized the National Security Agency to spy on calls between people in the U.S. and suspected terrorists abroad without obtaining court warrants. The administration said it needed to act more quickly than the court could and that the president had inherent authority under the Constitution to order warrantless domestic spying.
After the program was challenged in court, Bush last year put it under the supervision of the Foreign Intelligence Surveillance Court, established in 1978 after the domestic spying scandals of the 1970s.
"We think just as a common sense matter the legal theories for the president's wiretap programs cannot be classified and should be available to the public," said Marc Rotenberg, president of the Electronic Privacy Information Center, one of the groups seeking the memos.
"It's an important decision because up to this point the judge has relied on the government's assertion that it has done everything properly under the law and that it has disclosed everything it needs to disclose," Rotenberg said Saturday.
On the Net:
Court ruling: http://tinyurl.com/5c5g4j
Electronic Privacy Information Center: http://epic.org/
The National Security Archive: http://www.gwu.edu/(tilde)nsarchiv/
American Civil Liberties Union: http://www.aclu.com/
Justice Department: http://www.usdoj.gov/