Background: The burden and economic consequences of depression are high, mostly due to its recurrent nature. Due to current budget and time restraints, a preventive, low-cost, accessible minimal intervention is much needed. In this study, we evaluated the effectiveness of a supported self-help preventive cognitive therapy (S-PCT) added to treatment as usual (TAU) in primary care, compared to TAU alone. Methods: We conducted a randomized controlled trial among 248 patients with a history of depression, currently in full or partial remission or recovery. Participants were randomized to TAU augmented with S-PCT (n = 124) or TAU alone (n = 124). S-PCT consisted of an 8-week self-help intervention, supported by weekly telephone guidance by a counselor. The intervention included a self-help book that could be read at home. The primary outcome was the incidence of relapse or recurrence and was assessed over the telephone by the Structured Clinical Interview for DSM-IV axis 1 disorders. Participants were observed for 12 months. Secondary outcomes were depressive symptoms, quality of life (EQ-5D and SF-12), comorbid psychopathology, and self-efficacy. These secondary outcomes were assessed by digital questionnaires. Results: In the S-PCT group, 44 participants (35.5%) experienced a relapse or recurrence, compared to 62 participants (50.0%) in the TAU group (incidence rate ratio = 0.71, 95% CI 0.52-0.97; risk difference = 14, 95% CI 2-24, number needed to treat = 7). Compared to the TAU group, the S-PCT group showed a significant reduction in depressive symptoms over 12 months (mean difference-2.18; 95% CI-3.09 to-1.27) and a significant increase in quality of life (EQ-5D) (mean difference 0.04; 95% CI 0.004-0.08). S-PCT had no effect on comorbid psychopathology, self-efficacy, and quality of life based on the SF-12. Conclusions: A supported self-help preventive cognitive therapy, guided by a counselor in primary care, proved to be effective in reducing the burden of recurrent depression.