Williams v. Florida, 399 U.S. 78 (1970), is a United States Supreme Court case in which the Court held that the Fifth Amendment does not entitle a defendant in a criminal trial to refuse to provide details of his alibi witnesses to the prosecution, and that the Sixth Amendment does not require a jury to have 12 members.

Williams argued that the requirement to assist the prosecution in this way violated his Fifth Amendment right not to incriminate himself.

Also, in 1967 Florida had reduced the number of jurors in all non-capital cases from 12 to 6, and so Williams had been convicted by a jury of six.

The point of a jury trial was to prevent oppression by the government: Providing an accused with the right to be tried by a jury of his peers gave him an inestimable safeguard against the corrupt or overzealous prosecutor and against the compliant, biased, or eccentric judge.

[3]A companion case, Dunn v. Louisiana, was dismissed for lack of jurisdiction in a one-line per curiam opinion noting that Justice Marshall would have reversed for the reasons provided in his dissent in Williams.