This study aims to analyze the role of Fear of Missing Out (FOMO) as a mediating variable between social engagement and influencer marketing on the purchase decisions for smartphones with innovative features among Indonesian consumers. The main problem addressed is the lack of clarity on how digital marketing stimuli are converted into purchase decisions for high-involvement products in a competitive market characterized by feature homogenization. The objective is to examine the direct and indirect effects of these variables to provide an empirical model of modern consumer behaviour. This research uses a quantitative approach. Data was collected through an online questionnaire distributed to 100 Indonesian social media users, with the sample predominantly consisting of Gen Z and students. The data was analyzed using the Partial Least Squares Structural Equation Modeling (PLS-SEM) method with SmartPLS 4.0 software to test the relationships between the variables and the proposed hypotheses. The findings of this study indicate that FOMO has a strong and significant direct influence on purchase decisions, as does influencer marketing. Social engagement, however, does not have a significant direct effect. The most prominent conclusion is that FOMO acts as a significant and powerful mediator for both social engagement and influencer marketing. This reveals that FOMO is the key psychological mechanism that translates social media activities.