The production of free radicals in an organism is under the control of various antioxidant mechanisms. If their production overcomes the capacity of antioxidant protection, oxidative stress occurs which is capable of damaging different cellular structures and biomolecules, leading to various diseases. The importance of oxidative stress was proven in many psychiatric diseases among which are depression and bipolar disorder. Different studies show the significant improvement of clinical presentation when antioxidant substances are administered, suggesting that redox imbalance can influence their symptoms appearance and severity. In addition, oxidative stress is intercrossed with the different comorbidities that appear among depressive and bipolar patients. Beside the clinical presentation, oxidative stress influences the chronicity of depression, which was demonstrated in patients with recurrent depressive disorder. Better understanding of oxidant/antioxidant imbalance and its role in the pathophysiology of depression and bipolar disorder could be useful for the development of a novel therapeutic approach to the management of these diseases.