BackgroundDepression is a common mental disorder with a high social burden and significant impact on suicidality and quality of life. Treatment is often limited to drug therapies because of long waiting times to see psychological therapists face to face, despite several guidelines recommending that psychological treatments should be first-line interventions for mild to moderate depression.ObjectiveWe aimed to evaluate, among patients on a waitlist to receive secondary mental health care services for depression, how effective coach-guided web-based therapy (The Journal) is, compared with an information-only waitlist control group, in reducing depression symptoms after 12 weeks.MethodsWe conducted a randomized controlled trial with 2 parallel arms and a process evaluation, which included interviews with study participants. Participants assigned to the intervention group received 12 weeks of web-based therapy guided by a coach who had a background in social work. Patients in the control group receive a leaflet of mental health resources they could access. The primary outcome measure was a change in depression scores, as measured by the Patient-Health Questionnaire (PHQ-9).ResultsA total of 95 participants were enrolled (intervention, n=47; control, n=48). The mean change in PHQ-9 scores from baseline to week 12 was −3.6 (SD 6.6) in the intervention group and −3.1 (SD 6.2) in the control group, which was not a statistically significant difference with a two-sided alpha of .05 (t91=−0.37; P=.72, 95% CI −3.1 to 2.2). At 12 weeks, participants in the intervention group reported higher health-related quality of life (mean EuroQol 5 dimensions visual analogue scale [EQ-5D-VAS] score 66.8, SD 18.0) compared with the control group (mean EQ-5D VAS score 55.9, SD 19.2; t84=−2.73; P=.01). There were no statistically significant differences between the two groups in health service use following their initial consultation with a psychiatrist. The process evaluation showed that participants in the intervention group completed a mean of 5.0 (SD 2.3) lessons in The Journal and 8.8 (SD 3.1) sessions with the coach. Most participants (29/47, 62%) in the intervention group who completed the full dose of the intervention, by finishing 6 or more lessons in The Journal, were more likely to have a clinically important reduction in depressive symptoms at 12 weeks compared with the control group (Χ21=6.3; P=.01, Φ=0.37). Participants who completed the interviews reported that the role played by the coach was a major factor in adherence to the study intervention.ConclusionsThe results demonstrate that the use of guided web-based therapy for the treatment of depression is not more effective than information-only waitlist control. However, it showed that the coach has the potential to increase adherence and engagement with web-based depression treatment protocols. Further research is needed on what makes the coach effective.Trial RegistrationClinicalTrials.gov: NCT02423733; https://clinicaltrials.gov/ct2/show/NCT02423733