Background: Most interventions for depression have shown small or no effects. 'Third wave' cognitive therapy and mentalization-based therapy have both gained some ground as treatments of psychological problems. No randomised trial has compared the effects of these two interventions for patients with major depression.Methods/ design: We plan a randomised, parallel group, assessor-blinded superiority clinical trial. During two years we will include 84 consecutive adult participants diagnosed with major depressive disorder. The participants will be randomised to either 'third wave' cognitive therapy versus mentalization-based therapy. The primary outcome will be the Hamilton Rating Scale for Depression at cessation of treatment at 18 weeks. Secondary outcomes will be the proportion of patients with remission, Symptom Checklist 90 Revised, Beck's Depression Inventory, and The World Health Organisation-Five Well-being Index 1999.Discussion: Interventions for depression have until now shown relatively small effects. Our trial results will provide knowledge about the effects of two modern psychotherapeutic interventions.Trial registration: ClinicalTrials: NCT01070134.