Introduction: Psychotherapy is a first-line treatment for depression. However, capacities are limited, leading to long waiting times for outpatient psychotherapy in health care systems. Web-based interventions (WBI) could help to bridge this treatment gap. Objective: This study investigates the effectiveness of a guided cognitive-behavioral WBI in depressive patients seeking face-to-face psychotherapy. Methods: A 2-arm randomized controlled trial was conducted. Depressive patients (n = 136) recruited from the waiting lists of outpatient clinics were randomly assigned to an intervention group (IG; treatment as usual [TAU] + immediate access to WBI) or a control group (CG; TAU + access to WBI after follow-up). Depressive symptoms and secondary outcomes were assessed at baseline, 7 weeks, and 5 months after randomization. Results: Mixed-model analyses revealed a significant group × time interaction effect on depressive symptoms (F2, 121.5 = 3.91; p < 0.05). Between-group effect sizes were d = 0.55 at 7 weeks and d = 0.52 at 5 months. The IG was superior regarding psychological symptoms and mental health quality of life but not on physical health quality of life, attitudes, motivation for psychotherapy, or subjective need and uptake of psychotherapy. Conclusions: Patients waiting for face-to-face psychotherapy can benefit from a WBI when compared to TAU. Despite the reduction of depressive symptoms in the IG, the uptake of subsequent psychotherapy was still high in both groups. The effects remained stable at the 5-month follow-up. However, this study could not determine the proportion of specific intervention effects vs. nonspecific effects, such as positive outcome expectations or attention. Future research should focus on the long-term effects and cost-effectiveness of WBI before psychotherapy.