Consumer shopping patterns have altered due to Indonesia's e-commerce industry's explosive growth, allowing market participants to employ innovative marketing techniques. One widely-used strategy is flash sales, which capitalize on a sense of urgency through limited discount offers. Furthermore, Fear of Missing Out (FoMO) is increasingly impacting impulse shopping decisions, particularly in the digital realm, where consumers often feel pressured to make immediate purchases to avoid missing out on opportunities. With FoMO acting as a moderating variable in the relationship, this study examines how flash sales affect impulse buying made by Medan City Shopee users. We collected data from 132 active Shopee users in Medan City through questionnaires and interviews, and then we used structural equation modeling with SmartPLS for analysis. The analysis's findings demonstrate that flash sales significantly and favorably affect impulse buying. We also found that FoMO positively influenced impulse buying but did not mediate the relationship between flash sales and impulse buying. This finding suggests that, although FoMO encourages impulse buying independently, it does not strengthen the effect of flash sales. These results imply that e-commerce players need to consider the separate impacts of flash sale strategies and psychological factors such as FoMO when designing more effective marketing strategies.