Plessy v. Ferguson, 163 U.S. 537 (1896), was a landmark United States Supreme Court decision ruling that racial segregation laws did not violate the U.S. Constitution as long as the facilities for each race were equal in quality, a doctrine that came to be known as "separate but equal". The decision legitimized the many "Jim Crow laws" re-establishing racial segregation that had been passed in the American South after the end of the Reconstruction era in 1877.

The underlying case began in 1892 when Homer Plessy, a mixed-race man, deliberately boarded a whites-only train car in New Orleans. By boarding the whites-only car, Plessy violated Louisiana's Separate Car Act of 1890, which required "equal, but separate" railroad accommodations for white and black passengers. Plessy was charged under the Act, and at his trial his lawyers argued that judge John Howard Ferguson should dismiss the charges on the grounds that the Act was unconstitutional. Ferguson denied the request, and the Louisiana Supreme Court upheld Ferguson's ruling on appeal. Plessy then appealed to the U.S. Supreme Court.

In May 1896, the Supreme Court issued a 7–1 decision against Plessy, ruling that the Louisiana law did not violate the Fourteenth Amendment to the U.S. Constitution. It held that although the Fourteenth Amendment established the legal equality of whites and blacks, it did not and could not require the elimination of all "distinctions based upon color". The Court rejected Plessy's lawyers' arguments that the Louisiana law inherently implied that black people were inferior. It gave great deference to American state legislatures' inherent power to make laws regulating health, safety, and morals—the "police power"—and to determine the reasonableness of the laws they passed. Justice John Marshall Harlan was the lone dissenter from the Court's decision, writing that the U.S. Constitution "is color-blind, and neither knows nor tolerates classes among citizens", and so the laws distinguishing races should have been found unconstitutional.

Plessy is widely regarded as one of the worst decisions in U.S. Supreme Court history. Despite its infamy, the decision has never been overruled explicitly. Beginning in 1954 with Brown v. Board of Education, however, a series of the Court's later decisions have severely weakened Plessy to the point that it is usually considered de facto overruled.

In 1890, the Louisiana State Legislature passed a law called the Separate Car Act, which required separate accommodations for blacks and whites on Louisiana railroads. The law required passenger train officers to "assign each passenger to the coach or compartment used for the race to which such passenger belongs". The separation was purportedly done "to promote the comfort of passengers". The law also made it a misdemeanor for any passenger to "insist on going into a coach or compartment to which by race he does not belong," punishable by either a $25 fine or up to 20 days in prison.

A group of prominent black, creole of color, and white creole New Orleans residents formed a civil rights group called the Comité des Citoyens (Committee of Citizens). The group was dedicated to repealing the Separate Car Act and fighting its implementation. The Comité eventually persuaded Homer Plessy, a man of mixed race who was an "octoroon" (person of seven-eighths white and one-eighth black ancestry), to participate in an orchestrated test case to challenge the Act. Plessy had been born a free man and was fair-skinned. However, under Louisiana law, he was classified as black, and thus required to sit in the "colored" car.

On June 7, 1892, Plessy bought a first-class ticket at the Press Street Depot and boarded a "Whites Only" car of the East Louisiana Railroad in New Orleans, Louisiana, bound for Covington, Louisiana. The railroad company, which had opposed the law on the grounds that it would require the purchase of more railcars, had been previously informed of Plessy's racial lineage, and the intent to challenge the law. Additionally, the Comité des Citoyens hired a private detective with arrest powers to detain Plessy, to ensure that he would be charged for violating the Separate Car Act, as opposed to vagrancy or some other offense. After Plessy took a seat in the whites-only railway car, he was asked to vacate it, and sit instead in the blacks-only car. Plessy refused and was arrested immediately by the detective. As planned, the train was stopped, and Plessy was taken off the train at Press and Royal streets. Plessy was remanded for trial in Orleans Parish.

Plessy petitioned the state district criminal court to throw out the case, State v. Homer Adolph Plessy, on the grounds that the state law requiring East Louisiana Railroad to segregate trains had denied him his rights under the Thirteenth and Fourteenth amendments of the United States Constitution, which provided for equal treatment under the law. However, the judge presiding over his case, John Howard Ferguson, ruled that Louisiana had the right to regulate railroad companies while they operated within state boundaries. Four days later, Plessy petitioned the Louisiana Supreme Court for a writ of prohibition to stop his criminal trial.