Objectives Non-adherence to drugs is an obstacle getting in the way of treatment in all diseases, especially chronic diseases. This study aimed to investigate the adherence level of patients with psychiatric disorders and recognize factors contributing to patients' adherence to treatment. The study was done in Psychiatric clinics in Tehran, the capital city of Iran.Methods A cross-sectional study was carried out using a self-report method through a questionnaire consisting of 26 questions. A total of 429 people, who had referred to three large clinics during June and July 2017, participated in this study. In addition to socio-demographic, health and diagnostic characteristics of the patients, the adherence to psychiatric medication and some factors contributing to their adherence were reported. Key findings In this study, the majority of the patients with schizophrenia, that is 211 (49.2%), had low adherence, 141 (32.9%) had moderate adherence, and only 77 (17.9%) had full adherence to their treatment. According to the results of the questionnaire on Morisky drug adherence scale, most of the patients considered forgetfulness as a major cause of their non-adherence. Conclusion Non-adherence to treatment is a major problem in patients, and the intervention of pharmacists as the most accessible of medical staff as well as physicians can help to persuade patients to adhere and continually take medicines. Some issues such as the side effects of medications, patients' lack of belief in the need for treatment and patients' uncertainty about the effectiveness of medicines were the most important factors contributing to their non-adherent behaviour.