Background: The aim of this study was to investigate the role of psychological variables in medication adherence in OCD. Methods: This descriptive and correlational study was carried out at Tehran Institute of Psychiatry in Tehran. The statistical population of the present study includes all OCD patients referred to the Tehran Institute of Psychiatry. The participants were selected by available sampling method. The patients completed the demographic questionnaire, Yale Brown Obsessive-Compulsive Disorder Scale-Second Edition (Y-BOCS-II), Vancouver Obsessional Compulsive Inventory (VOCI), Medication Adherence Rating Scale (MARS), Drug Attitude Questionnaire (DAI-10), Multidimensional Scale of Perceived Special Support (MSPSS), Coping strategies scale of Lazarus and Folkman (WOCQ), and Temperament and Character Inventory (TCI). Multiple Linear Regression (MLR) was used to analyze the data. Results: The variable of education status (r=0.18) had a positive relationship and the variable of hospitalization history (r=-0.26) had a negative relationship with medication adherence. Medication adherence is only negatively associated with obsessive thoughts (r=- 0.18, r=-0.20). Duration of drug use (r=0.27), attitude to treatment (r=0.25) and social support (r=0.54) had a positive relationship with medication adherence. Among the various dimensions of temperament and character, four dimensions of harm avoidance (r=-0.29), reward dependence (r=-0.44), persistence (r=-0.20) and self-transcendence (r=0.32) had a significant correlation with medication adherence. Variables of social support, reward dependence, persistence, harm avoidance and education status had the highest regression effect on medication adherence. Conclusion: Medication adherence is one of the behaviors that predicts successful treatment and reduces the negative side effects and severity of the OCD.