In political science, a reactionary or a reactionist is a person who holds political views that favor a return to the status quo ante, the previous political state of society, which that person believes possessed positive characteristics absent from contemporary society. As a descriptor term, reactionary derives from the ideological context of the left–right political spectrum. As an adjective, the word reactionary describes points of view and policies meant to restore a past status quo ante.

In ideology, reactionism is a tradition in right-wing politics; the reactionary stance opposes policies for the social transformation of society, whereas conservatives seek to preserve the socio-economic structure and order that exists in the present. In popular usage, reactionary refers to a strong traditionalist conservative political perspective of a person opposed to social, political, and economic change.

Reactionary ideologies can be radical in the sense of political extremism in service to re-establishing past conditions. In political discourse, being a reactionary is generally regarded as negative; Peter King observed that it is "an unsought-for label, used as a torment rather than a badge of honor." Despite this, the descriptor "political reactionary" has been adopted by writers such as the Austrian monarchist Erik von Kuehnelt-Leddihn, the Scottish journalist Gerald Warner of Craigenmaddie, the Colombian political theologian Nicolás Gómez Dávila, and the American historian John Lukacs.