A grandfather clause (or grandfather policy) is a provision in which an old rule continues to apply to some existing situations while a new rule will apply to all future cases. Those exempt from the new rule are said to have grandfather rights or acquired rights.

(Often, such a provision is used as a compromise or out of practicality, to effect new rules without upsetting a well-established logistical or political situation. This extends the idea of a rule not being retroactively applied.)

The term originated in late nineteenth-century legislation and constitutional amendments passed by a number of U.S. Southern states, which created new requirements for literacy tests, payment of poll taxes, and/or residency and property restrictions to register to vote. States in some cases exempted those whose ancestors (grandfathers) had the right to vote before the Civil War, or as of a particular date, from such requirements. The intent and effect of such rules was to prevent poor and illiterate African-American former slaves and their descendants from voting, but without denying poor and illiterate whites the right to vote. Although these original grandfather clauses were eventually ruled unconstitutional, the terms grandfather clause and grandfather have been adapted to other uses.