Introduction: Fear of Missing Out (FoMO) is a pervasive apprehension about missing rewarding experiences that others might be having, typically accompanied by a strong drive to remain constantly connected. Prior work links FoMO to regulatory vulnerabilities and problematic digital behaviors, often positioning FoMO as a mechanism translating dispositional risk into maladaptive smartphone use. However, no studies to date have examined reflective functioning, described as the capacity to comprehend one's own and others' mental states, as a potential protective moderator of FoMO's impact on behavior. Purpose: This study explored the relationships among FoMO, psychological distress, problematic smartphone use, and reflective functioning (both certainty and uncertainty about mental states) in a sample of emerging adults, with the aim of clarifying how individual differences in reflective functioning capacities may shape vulnerability to maladaptive digital behaviors. Method: An online survey disseminated through social network platforms was completed by 322 emerging adults aged 18 to 35 years (Mage = 24.83, SD = 3.96). Participants provided sociodemographic information and completed a standardized set of self-report questionnaires, including the Depression Anxiety Stress Scales-21, the Online FoMO Scale, the Reflective Functioning Questionnaire (certainty and uncertainty subscales), and the Smartphone Addiction Inventory. Results: FoMO showed a strong positive association with problematic smartphone use, and moderate positive associations with psychological distress and uncertainty about mental states. Conversely, FoMO correlated negatively with certainty about mental states and with age. Conclusions: These findings highlighted that both reflective functioning dimensions showed associations with FoMO; specifically, greater uncertainty about mental states emerged as a significant correlate of FoMO, suggesting that difficulties in reflective functioning may intensify the tendency to remain constantly connected. Targeting reflective uncertainty may thus represent a valuable direction for interventions designed to reduce FoMO and its maladaptive behavioral outcomes.