Introduction: Both imagery rescripting and imaginal exposure have been proven to be effective in the treatment of chronic nightmares when compared to a waitlist condition. Little is known about their comparative efficacy and their efficacy compared to an active control. Objective: The aims of this study were to compare the two treatments to one another and to positive imagery as an active control, and to explore covariates of the treatment effect. Methods: In this single-blinded randomized controlled trial, 96 patients with nightmare disorder (idiopathic nightmares) from an outpatient clinic were randomly assigned to a single individual treatment session of rescripting, exposure, or positive imagery and 4 weeks of practice at home. The primary outcome was nightmare distress, and the secondary outcomes were nightmare frequency, nightmare effects, self-efficacy, and general psychopathology. Results: Nightmare distress was reduced in all groups (imagery rescripting: Cohen’s d = –1.04, imaginal exposure: d = –0.68, positive imagery: d = –0.57), as were nightmare frequency, nightmare effects, and psychopathology. Self-efficacy was enhanced. No differential treatment effects were found on any primary or secondary measure. Treatment gains were not associated with demographic or disorder characteristics, baseline values, treatment credibility, or the number of practice sessions. Conclusions: Even short nightmare treatments are effective regardless of personal characteristics, and different interventions produce similar results. Future research should aim to clarify the mechanisms of action. Health care should make more use of these powerful and easy-to-administer nightmare treatments.