BACKGROUND: Common mental disorders (CMD) are a leading cause of disability globally. Emerging evidence indicates that in low and middle income countries the treatment gap for CMD can be addressed through the use of trained and supervised lay health workers (LHWs). Few clinical trials have evaluated the use of such task-shifting approaches in sub-Saharan Africa. In Zimbabwe, we have successfully piloted a task-shifting intervention delivered by LHWs. This protocol describes a cluster randomised controlled trial to assess the effectiveness of this intervention. METHODS: Each of 24 randomly selected clinics from a pool of 42 in Harare will recruit 24 participants (N = 576). The clinics are randomised in a 1:1 ratio to receive either the intervention package [a problem solving therapy package delivered over a 4-6 week period by LHWs (N = 24) followed by a 6-week group support programme which focuses mainly on teaching a craft skill] or enhanced usual care, which includes usual care and psycho-education. Primary care attenders aged 18 years and above who score positive on a locally validated CMD screening questionnaire (Shona Symptom Questionnaire, SSQ-14) will be eligible for recruitment and asked for informed consent to participate in the trial. The primary measure is the SSQ score at 6 months. CONCLUSION: This effectiveness trial using LHWs to address the treatment gap for CMD will contribute to the body of knowledge on the feasibility and ability for scale-up of interventions for CMD. TRIAL REGISTRATION: PACTR201410000876178.