"Guinn v. United States" was a landmark U.S. Supreme Court case decided in 1915 that struck down the "grandfather clause," a provision in Southern state laws that exempted people from literacy tests and poll taxes if their grandfathers had been eligible to vote before the Civil War. These clauses were designed to disenfranchise African Americans and poor whites who were not able to meet the voting requirements.

In the case, the Supreme Court declared the grandfather clauses unconstitutional as a violation of the Fifteenth Amendment, which prohibits states from denying the right to vote based on race. This decision was a crucial victory for civil rights and voting rights for African Americans during the Jim Crow era. It marked one of the first significant legal victories in the fight against racial discrimination in voting practices in the United States.