Which U.S. Supreme Court case held that the Fourth Amendment requires a warrant for wiretapping?

Katz v. United States held that the Fourth Amendment requires a warrant for the government’s wiretapping of a private telephone conversation, absent a recognized exception.

Charles Katz used a public phone booth in Los Angeles to transmit sports-wagering information. FBI agents recorded his calls with a listening device attached outside the booth, and they had no warrant. Katz argued that the recordings should be excluded, but lower courts treated the absence of physical trespass as decisive.

In 1967, the Supreme Court rejected that narrow approach. The Court held that the Fourth Amendment protects people, not merely physical places, and that Katz reasonably expected privacy in his conversation. The warrantless recordings were therefore an unconstitutional search. Justice John Marshall Harlan’s concurrence later supplied the influential two-part “reasonable expectation of privacy” test.

A common oversimplification is that Katz created an absolute warrant requirement for every wiretap. The decision made electronic surveillance subject to Fourth Amendment scrutiny, but lawful warrants and recognized exceptions can still permit some surveillance. Katz also overturned the older trespass-focused approach of Olmstead v. United States.