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 the person believes possessed positive characteristics that are 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 status quo ante.

As an 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. To some writers, the term reactionary carries negative connotations—Peter King observed that it is “an unsought-for label, used as a torment rather than a badge of honor.”