Lankford v. Idaho, 500 U.S. 110 (1991), was a United States Supreme Court case in which the Court held that the petitioner, Bryan Lankford, had been unconstitutionally sentenced to death in violation of the Due Process Clause of the Fourteenth Amendment. The Court held 5–4 that Lankford had not received adequate notice that he could be sentenced to death before the trial judge imposed such a sentence on him, and therefore reversed the prior ruling to the contrary by the Idaho Supreme Court.

Bryan Lankford, along with his brother Mark Lankford, was convicted of the double murder of Robert and Cheryl Bravence, a couple from Texas vacationing in Idaho. The murder took place in 1983, and the Lankford brothers were convicted of it the following year.^[1] At Bryan Lankford's trial, the prosecutor had explicitly told the judge that he would not seek the death penalty, and it was not discussed during the subsequent sentencing hearing. Nevertheless, the following week, the judge sentenced both Bryan and Mark Lankford to death, and the Idaho Supreme Court upheld both brothers' death sentences. Both brothers then appealed their sentences to the United States Supreme Court, which granted certiorari in Bryan's case while denying it in Mark's case.^[2]

The majority opinion was written by Justice John Paul Stevens and joined by Thurgood Marshall, Harry Blackmun, Sandra Day O'Connor, and Anthony Kennedy. The dissenting opinion was written by Justice Antonin Scalia and joined by Chief Justice William Rehnquist, as well as by Byron White and David Souter. Because Justice O'Connor was in the majority, this case marked the first one of the 1990–1991 term in which Souter and O'Connor were on opposite sides.^[2][3]

After the Supreme Court's decision in Lankford v. Idaho, Bryan Lankford's sentence was commuted from death to life in prison. As of 2018, he remains incarcerated at Idaho State Correctional Center in Boise.^[1][4] The case also created what has become known as the "Lankford notice exception", which states that if, after an arraignment of a criminal defendant, a prosecutor indicates to the judge that they will not seek the death penalty, it is an unconstitutional violation of due process to impose the death penalty on that defendant.^[5]