A decision exhibits reactance if it is not directly welfare maximizing but represents a way to protest against a threat to the decision maker's freedom of choice. We provide a first axiomatic revealed preference characterization of this phenomenon, which yields necessary and sufficient conditions for deviations from rational choice to be ascribed to reactance. These conditions are shown to characterize a representation of preferences underlying choices consistent with reactance. We next look at two applications that have been (informally) associated with reactance in the psychology literature and demonstrate that reactance can imply the emergence of conspiracy theories and a backlash of integration policy targeted towards immigrants. Finally, we derive the resulting preference ordering over opportunity sets for agents whose final choices are consistent with reactance.