This study examines how the amendment power can be used to legitimately produce a constitutional revolution, altering the core identity of a constitutional system. In doing so, I introduce the concept of the revolutionary amendment and discuss how such an amendment can achieve legitimacy in a constitutional system. Drawing on deliberative civic republican theory, I argue that the process of enactment must approximate the primary constituent power by fostering citizen representation and deliberation in both the drafting and the ratification of the amendment. This approximation thesis can help determine when the citizens of the state will see a revolutionary amendment as legitimate. This theoretical contribution is followed by case studies of contemporary constitutional revolutions in Ireland and the United Kingdom.