Lower courts had repeatedly found parts of the material support law unconstitutionally vague in a lawsuit that began in the late 1990s.
Despite the risk of prosecution, Fertig said he would continue his work on behalf of the Kurds. "We will not let it inhibit our commitment to the Kurdish people," he said.
In his dissent, Breyer recognized the importance of denying money and other resources to terrorist groups. "But I do dispute whether the interest can justify the statute's criminal prohibition."
Breyer said the aid groups' mission is entirely peaceful and consists only of political speech, including how to petition the U.N.
But Roberts said the U.N. was forced to close a refugee camp in northern Iraq, near the Turkish border, because it had come under PKK control.
"Training and advice on how to work with the United Nations could readily have helped the PKK in its efforts to use the United Nations camp as a base for terrorist activities," Roberts said.
The other justices in the majority were Samuel Alito, Anthony Kennedy, Antonin Scalia and Clarence Thomas.
The cases are Holder v. Humanitarian Law Project, 08-1498, and Humanitarian Law Project v. Holder, 09-89.