Plessy v. Ferguson, 163 U.S. 537 (1896), was a landmark decision of the United States Supreme Court in which the Court ruled 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 state laws re-establishing racial segregation that had been passed in the American South after the end of the Reconstruction Era (18651877).

The underlying case began in 1892 when Homer Plessy, a mixed-race resident of New Orleans, deliberately violated Louisiana's Separate Car Act of 1890, which required "equal, but separate" railroad accommodations for white and non-white passengers. Plessy was charged with boarding a "whites-only" car, and his lawyers petitioned Judge John Howard Ferguson to throw out the case on the grounds that the law was unconstitutional. Ferguson overruled Plessy's petition, and the Louisiana Supreme Court upheld Ferguson's ruling. Plessy then appealed to the U.S. Supreme Court.

In May 1896, the Supreme Court issued a 71 decision against Plessy, ruling that the Louisiana law did not violate the Fourteenth Amendment to the U.S. Constitution and stating 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, and gave great deference to American state legislatures' inherent power to make laws regulating health, safety, and moralsthe "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 law's distinguishing of passengers' 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 explicitly overruled. But a series of the Court's later decisions, beginning with the 1954 decision Brown v. Board of Educationwhich held that the "separate but equal" doctrine is unconstitutional in the context of public schools and educational facilitieshave severely weakened Plessy to the point that it is considered to have been de facto overruled.

Homer Adolph Plessy (born Homère Patris Plessy; 1862 or March 17, 1863 – March 1, 1925) was an American shoemaker and activist, best known as the plaintiff in the United States Supreme Court decision Plessy v. Ferguson. He staged an act of civil disobedience to challenge one of Louisiana's racial segregation laws and bring a test case to force the U.S. Supreme Court to rule on the constitutionality of segregation laws. The Court decided against Plessy. The resulting "separate but equal" legal doctrine determined that state-mandated segregation did not violate the Fourteenth Amendment to the United States Constitution as long as the facilities provided for both black and white people were putatively "equal". The legal precedent set by Plessy v. Ferguson would last into the mid-20th century, until a series of landmark Supreme Court decisions gradually set new precedents concerning segregation, beginning with Brown v. Board of Education in 1954.

Plessy was born a free person of color in a family of French-speaking Louisiana Creole people. Growing up during the Reconstruction era, Plessy lived in a society in which black children attended integrated schools, black men could vote, and interracial marriage was legal. However, many of those civil rights eroded following the withdrawal of U.S. federal troops from the former Confederacy in 1877. In the 1880s, Plessy became involved in political activism, and in 1892, the civil rights group Comité des Citoyens recruited him for an act of civil disobedience to challenge Louisiana's Separate Car Act, which required separate accommodations for black and white people on railroads. On June 7, 1892, Plessy purchased a ticket for a "whites only" first-class train coach, boarded the train, and was arrested by a private detective hired by the group. Judge John Howard Ferguson ruled against Plessy in a state criminal district court, upholding the law on the grounds that Louisiana had the right to regulate railroads within its borders. Plessy appealed to the U.S. Supreme Court, which heard the case four years later in 1896 and ruled 7–1 in favor of Louisiana, establishing the "separate but equal" doctrine as a legal basis for the Jim Crow laws that would remain in effect into the 1950s and 1960s.