Depression is a severe and heterogeneous disorder resulting from interacting genetic, environmental, and epigenetic factors. Nutrient deficiency resulting from bariatric bypass surgery has been involved in the pathophysiological mechanisms of depression and treatment response.We report the case of a patient who developed, after a bariatric bypass surgery, a severe depressive episode, refractory to both pharmacological treatment and electroconvulsivotherapy (ECT). Folate deficiency was evidenced. A dramatic response to ECT was observed after folate supplementation. We focused on the involvement of folate in the pathophysiological mechanisms of depression and response to both pharmacological treatment and ECT. We emphasize on the need for close monitoring of patients experiencing psychiatric disorder (in particular, depressive unipolar disorder) after bariatric surgery.