The ruling released Monday by the 9th U.S. Circuit Court of Appeals in San Francisco affirms a 2005 decision by a lower court judge.
That judge, Audrey Collins, ruled on a petition seeking to clear the way for U.S. groups and individuals to assist organizations in Turkey and Sri Lanka with training on applying for disaster relief or conducting peace negotiations.
Collins said language in the Patriot Act was vague on matters involving training, expert advice or assistance, personnel and service to foreign terrorist organizations.
Without clear language, the plaintiffs argued, those who provide assistance to those organizations could be subject to U.S. prison terms of up to 15 years.
Charles Miller, a U.S. Justice Department spokesman, said his agency was reviewing Monday's ruling to determine a response.
Collins' ruling prevented the federal government from enforcing the vague Patriot Act provisions as they apply to the terrorist groups named in the lawsuit.
In its 27-page decision, the 9th U.S. Circuit Court of Appeals held that to survive a vagueness challenge, a statute "must be sufficiently clear to put a person of ordinary intelligence on notice that his or her contemplated conduct is unlawful."
The language covered by the ruling remained unconstitutionally vague despite Congressional amendments to the Patriot Act meant to remedy the problems, the appeals court ruled.