Introduction: Obesity is a multifactorial disease that is increasing in developing countries. People with obesity suffer from quality of life (QoL)-related health problems. The purpose of this study was to investigate the effect of gastric bypass surgery on three aspects of QoL, anxiety, and depression among patients with obesity. Method: This is a cross-sectional study on all patients with obesity referred to Sina Hospital during the three months of Summer 2016. Data collection was done by means of the Beck Depression Inventory, Beck Anxiety Inventory, and SF-36 Questionnaire. Data were analyzed using paired t-test and Pearson correlation coefficient using IBM SPSS 20.0 software. The significance level was considered as 5%. Results: The mean score of the QoL questionnaire and its dimensions in patients with obesity before and three months after surgery was significantly different (p<0.001). Depression and anxiety of the patients also significantly decreased compared to the pre-operation (p<0.001). There was a significant correlation between decreased anxiety and depression and improvement in the physical and mental health of patients after the operation (p<0.001). Conclusion: Gastric bypass surgery leads to improved QoL, decreased anxiety, and depression in patients with obesity. Correlation between anxiety and depression with components of QoL also showed improvement in patients' life after gastric bypass surgery.