Guinn v. United States was a landmark legal case in Black history that was decided by the U.S. Supreme Court in 1915. The case challenged the constitutionality of the "grandfather clause," a Jim Crow-era law designed to disenfranchise Black voters in Southern states.

The grandfather clause stated that individuals could only vote if their grandfathers had been eligible to vote before the Civil War, effectively excluding most Black people from voting since their ancestors were enslaved and not permitted to vote. The Supreme Court ruled in Guinn v. United States that the grandfather clause was unconstitutional under the Fifteenth Amendment, which guaranteed the right to vote regardless of race, color, or previous condition of servitude.

This decision was a significant victory for civil rights and helped pave the way for future legal challenges to discriminatory voting practices. Guinn v. United States played a key role in the ongoing struggle for voting rights for African Americans in the United States.