However, the justices left in place an injunction against the implementation of the act, and sent the challenge to the law back to the lower courts to explore other constitutional issues.
The limited and temporary victory comes less than a month after another government weapon against online pornography, the 1996 Child Pornography Prevention Act, was struck down by the high court as unconstitutionally overbroad.
In last month's ruling, a 6-3 court majority said the law could not be applied against images in which no children were actually used -- such as computer-generated images or images of adults posing as children.
The majority also said the law, if not struck down, could have been unconstitutionally applied to such works of art as Romeo and Juliet.
Monday, Justice Clarence Thomas delivered the opinion of the court upholding the "community standards" provisions of COPA. No other justice signed on to every provision of his opinion, but seven court members agreed with sections of it.
Only Justice John Paul Stevens dissented from the final judgment.
The battle lines between the federal government and such organizations as the American Civil Liberties Union were first drawn over the 1996 Communications Decency Act.
The act prohibited the "knowing" transmission of "indecent" messages over the Internet to anyone younger than 18.
The ACLU and others challenged the CDA in court. Eventually, the Supreme Court said the government had a compelling interest in protecting minors from "indecent" material online -- material that was less offensive than obscenity -- but ruled that the CDA was not the least restrictive way to accomplish that goal, and struck the law down as unconstitutional.
In any case involving the free-speech provisions of the First Amendment, the courts have required the government to show that it is taking the least restrictive way to legitimately protect the public.
With the CDA in shreds, Congress enacted the Child Online Protection Act and President Clinton signed it into law in 1998. COPA includes civil and criminal penalties for anyone who "knowingly ... by means of the World Wide Web, makes any communication for commercial purposes that is available to any minor and that includes any material that is harmful to minors."
It defined those type of materials as what "the average person, applying contemporary community standards, would find ... is designed to appeal to, or is designed to pander to, the prurient interest," in which there is some sexual element and which "lacks serious literary, artistic, political or scientific value for minors."
The law said an "affirmative defense" to prosecution would be a "good faith" attempt to keep the material from minors.
An ACLU-led coalition again challenged the law, and a federal judge issued a preliminary injunction against its implementation. Eventually, a federal appeals court in Philadelphia agreed with the judge, saying COPA was probably unconstitutional.
The appeals court said COPA's reliance on community standards "in and of itself, imposes an impermissible burden on constitutionally protected First Amendment speech."
The community standards threshold "as applied to the Internet means than any communication available to a nationwide audience will be judged by the standards of the community mostly likely to be offended by the message," the court said.
That meant that Web publishers would need to either severely censor themselves or establish some kind of age or credit-card verification for material that "might be deemed harmful by the most puritan of communities in any state."
Following the appeals court ruling, the Bush administration asked the Supreme Court to take the case and settle the community standards issue.
Writing for the court in Monday's opinion, Thomas said, "The scope of our decision today is quite limited. We hold only that COPA's reliance on community standards to identify 'material that is harmful to minors' does not by itself render the statute substantially overbroad for the purposes of the First Amendment."
Thomas said the Supreme Court was not expressing any view as to whether COPA is overbroad for other reasons, whether it is unconstitutionally vague or whether it could survive the "strict scrutiny" standard that courts apply when assessing restrictions on free speech.
Those issues should first be explored by the appeals court, Thomas said, sending the case back down for another hearing.
The government "does not ask us to vacate the preliminary injunction entered by the district court (trial judge)," Thomas said, "and in any event, we could not do so without addressing matters yet to be considered by the court of appeals."
The injunction stays put, he added, until further action by the appeals court or the trial judge.
In his dissent, Stevens warned that "community standards" have been used to protect free speech in past trials, with individual jurors being told that they could not substitute their own prejudices for the judgment of the community as a whole.
"In the context of the Internet, however, community standards become a sword, rather than a shield," Stevens said. "If a prurient appeal is offensive in a puritan village, it may be a crime to post it on the World Wide Web. The Child Online Protection Act ... restricts access to adults as well as children to materials that are 'harmful to minors.'"
(No. 00-1293, Ashcroft vs. ACLU et al.)