The present study investigates the association between Fear of Missing Out (FoMO) and Problematic Internet Use (PIU) in a sample of 358 cisgender emerging adults (74.58% females assigned at birth; Mage = 25.02, SD = 2.60; age range: 18–29 years), with a specific focus on the roles of various stances of Epistemic Trust, including Trust, Mistrust, and Credulity. The aim is to explore the complexities of these relationships and their implications for the psychological well-being of this population. We computed a mediation model to examine the relationships among PIU as the dependent variable, FoMO as the predictor, and Epistemic Trust, Mistrust, and Credulity as the mediating factors. Covariates such as age, gender, and sexual orientation were also considered in the analysis. The results revealed significant indirect effects for both Mistrust and Credulity. Specifically, higher levels of FoMO were associated with increased Mistrust and Credulity, leading to greater PIU. In contrast, the indirect effect of Trust was not significant. Also, FoMO had a direct effect on PIU. The results highlight the importance of considering Mistrust and Credulity as potential risk factors for Internet addiction in emerging adults. These findings have practical implications for clinical practice, psychological assessment, and intervention strategies, emphasizing the need to address FoMO and its associated vulnerabilities within different therapeutic settings. By doing so, mental health professionals can better support the psychological well-being of emerging adults and assist them in navigating the challenges inherent to this crucial developmental stage.