BACKGROUND: Depression constitutes a significant public health burden and is associated is with high level of individual suffering. Insufficient human and material resources impede the provision of adequate care for persons with the condition in low- and middle-income countries. It is commonly recognized that, to bridge this treatment gap, it is essential to integrate the treatment of depression into primary health care system. METHODS/DESIGN: STEPCARE is a two-arm parallel cluster randomized controlled trial to compare a stepped-care intervention package for depression in primary health care with care as usual in Nigeria. Randomization was conducted at the level of the participating primary health care clinics, while interventions are delivered to consenting individual participants who screen positive on the 9-item patient health questionnaire (PHQ-9 score ≥ 11) and fulfil the DSM-IV criteria for major depression. Intervention delivered by trained primary health care workers (PHCW) supported by general physicians and psychiatrists as needed is in 3 steps determined by response to treatment. Each step consists of psychological interventions (including psychoeducation, activity scheduling, social network reactivation and problem solving treatment) offered to all participants and, depending on severity and response, medication. Primary outcome, assessed at 12 months following recruitment into the trial, is recovery from depression as shown by a PHQ-9 score of less than 6. Secondary outcomes include changes in disability, quality of life and service utilization assessed at 6 and 12 months. DISCUSSION: The stepped care model examines the effectiveness of an intervention package for depression in which the intensity of treatment is determined by the clinical need of the patients. This approach is designed to make the most efficient use of available resources. TRIAL REGISTRATION: ISRCTN46754188 (ISRTCN registry at isrtcn.com; registered 23 September 2013).