Napue v. Illinois, 360 U.S. 264 (1959), was a United States Supreme Court case in which the Court held that the knowing use of false testimony by a prosecutor in a criminal case violates the Due Process Clause of the Fourteenth Amendment to the United States Constitution, even if the testimony affects only the credibility of the witness and does not directly relate to the innocence or guilt of the defendant.

In 1935, the Supreme Court briefly wrote in Mooney v. Holohan[2] that prosecutors violate the Due Process Clause if they knowingly present perjured testimony.

However, in Alcorta, the Court refrained from setting a specific standard regarding when false testimony becomes material enough to warrant reversal of a conviction.

[3][7] After Napue's conviction, the Assistant State's Attorney who prosecuted the murder filed a petition to reduce Hamer's sentence.

[8] On the basis of the Assistant State's Attorney's testimony, the trial court denied the petition to overturn Napue's conviction.

The Court reiterated that prosecutors have a duty to seek to correct false testimony when it occurs in order for a trial to be fair, and held that any false testimony allowed by the prosecutor that may affect the outcome of trial creates a violation of due process rights protected by the Fourteenth Amendment to the United States Constitution.

In Giglio, the Court also decided the threshold for materiality for Napue claims, holding: "A new trial is required if the false testimony could [...] in any reasonable likelihood have affected the judgment of the jury".