The landmark Supreme Court case that ruled that "separate but equal" was unconstitutional was Brown v. Board of Education, decided in 1954. This pivotal ruling overturned the precedent established in Plessy v. Ferguson (1896), which allowed racial segregation. The Court's decision held that racial segregation in public schools was inherently unequal, violating the Equal Protection Clause of the Fourteenth Amendment. This historic case was a significant milestone in the Civil Rights Movement, catalyzing further efforts toward desegregation and racial equality in the United States.