Background and aimsPsychodynamic psychotherapy (PDT) is an effective treatment for major depressive disorder (MDD), but not all clients with MDD can receive psychotherapy. Using the Internet to provide psychodynamic treatments is one way of improving access to psychological treatments for MDD. The aim of this randomised controlled trial was to investigate the efficacy of an Internet-based psychodynamic guided self-help treatment for MDD.MethodsNinety-two participants who were diagnosed with MDD according to the Mini-International Neuropsychiatric Interview were randomised to treatment or an active control. The treatment consisted of nine treatment modules based on psychodynamic principles with online therapist contact. The active control condition was a structured support intervention and contained psychoeducation and scheduled weekly contacts online. Both interventions lasted for 10 weeks. The primary outcome measure was the Beck Depression Inventory-II (BDI-II).ResultsMixed-effects model analyses of all randomised participants showed that participants receiving Internet-based PDT made large and superior improvements compared with the active control group on the BDI-II (between-group Cohen's d = 1.11). Treatment effects were maintained at a 10-month follow-up.ConclusionsInternet-based psychodynamic guided self-help is an efficacious treatment for MDD that has the potential to increase accessibility and availability of PDT for MDD.Trial RegistrationClinicaltrials.gov: NCT01324050