The principal aim of this study was to investigate the psychosocial well-being of emerging adults using psychological states associated with this transitional phase and classic measures of emerging adulthood. We expected psychological states to be more closely associated with psychological well-being than classic markers of achieved adulthood. Data were collected in the Cohort Study on Substance Use Risk Factors from 4,991 Swiss men aged 18-25 years. The assessment included the Short Form of the Inventory of Dimensions of Emerging Adulthood (IDEA-8), classic markers of achieved adulthood (e.g., financial independence, stable relationship), and psychosocial well-being. Structural equation models (SEMs) were conducted to test the association between measures of emerging adulthood and psychosocial well-being. Overall, the results highlighted contrasting associations of measures of emerging adulthood and psychosocial well-being. Youths facing negative psychological states (dimension "negativity") and exploring life without knowing how to define themselves (dimension "identity exploration") had a decreased psychosocial well-being. On the contrary, youths exploring many opportunities with an optimistic perspective (dimension "experimentation") had an increased psychosocial well-being. By contrast, classic markers of adulthood were less related to psychosocial well-being. The IDEA-8 Scale appeared to be a useful screening tool for identifying vulnerable youths, and emerging adulthood should be measured with a focus on the psychological states associated with this period. This information may be valuable for mental health systems that have not yet adapted to emerging adults' needs.