Background: Multiple Sclerosis (MS) affects various aspects of a patient's life. This study aimed to investigate the effect of positive thinking training on the quality of life and emotion regulation among patients with MS. Methods: This research was a quasi-experimental study conducted on a sample of 40 patients selected by convenience sampling method. Eight 90-minute sessions of positive thinking training were held for the experimental group for two months. The Post-test and follow-up were conducted at the end of the sessions and a month later, respectively. Ware and Sherbourne Quality of Life Questionnaire (1999) and Ibanez et al. Self-Regulation Scale (2005) were used for collecting the data. The data analyses were conducted using the analysis of covariance. The inclusion criteria were suffering from MS and being able to read and write. The exclusion criteria were having mental disorders, cognitive disorders, a history of substance addiction, and other neurological disorders. Results: The results demonstrated a significant difference between the scores obtained by the groups with regard to emotion regulation (Eta-squared=0.62, P<0.001). Besides, considering physical health, there was a significant difference between the scores obtained by the groups (Eta-squared=0.53, P<0.001), and considering mental health, there was a significant difference between the scores obtained by the groups (Eta-squared=0.74, P<0.001). Conclusion: These results indicated that positive thinking training was effective in improving emotion regulation and promoting quality of life in pre-test, post-test, and follow-up. Authorities are recommended to conduct programs based on positive thinking training to promote emotion regulation and increase MS patients' quality of life. It is recommended that training positive thinking skills be conducted as a regular program at associations and organizations, which support MS patients to promote the levels of quality of life and emotion regulation among these patients.