WASHINGTON—National Security Agency spy activities on at least one occasion have violated the Fourth Amendment protections against unreasonable search and seizure, according to a ruling by the U.S.'s secret national security court.

The ruling, which was revealed Friday in a declassified statement, represented the first time the government has acknowledged U.S. spy activities violated the constitution since the passage of a 2008 law that overhauled surveillance laws following the uproar over the NSA's warrantless wiretapping...