Grandfather clause, statutory or constitutional device enacted by seven Southern states between 1895 and 1910 to deny suffrage to African Americans. It provided that those who had enjoyed the right to vote prior to 1866 or 1867, and their lineal descendants, would be exempt from recently enacted educational, property, or tax requirements for voting. Because the former slaves had not been granted the franchise until the adoption of the Fifteenth Amendment in 1870, those clauses worked effectively to exclude black people from the vote but assured the franchise to many impoverished and illiterate whites.

Although the U.S. Supreme Court declared in 1915 that the grandfather clause was unconstitutional because it violated equal voting rights guaranteed by the Fifteenth Amendment, it was not until Pres. Lyndon B. Johnson introduced the Voting Rights Act of 1965 that Congress was able to put an end to the discriminatory practice. The act abolished voter prerequisites and also allowed for federal supervision of voter registration. With the passage of the Voting Rights Act, the Fifteenth Amendment was finally enforceable.