The present study investigated the relationship between optimism and happiness and the mediating role of lifestyle. The research method was descriptive correlation. To conduct the study, 155 religious students of Qom that were selected through available sampling answered three instruments that were Noori’s Islamic optical optimism questionnaire, Oxford happiness questionnaire, and Kaviani’s Islamic lifestyle questionnaire. In data analysis, path analysis method was used. Findings showed that: a) There was a significant positive relationship between optimism and happiness b) There was a significant positive relationship between optimism and Islamic lifestyle c) There was a significant positive relationship between Islamic lifestyle and happiness d) The results of path analysis showed that optimism through Islamic lifestyle has an effect on happiness and 0.33 of the effect of optimism on happiness is mediated and predicted by Islamic lifestyle. Therefore, strengthening Islamic optimism with the focus on improving the Islamic way of life promotes happiness.