Background <p>Psychological flourishing is linked to numerous positive adolescent outcomes, yet reliable assessment tools remain limited. The EPOCH Measure of Adolescent Well-Being captures five flourishing dimensions: Engagement, Perseverance, Optimism, Connectedness, and Happiness. Although validated in several languages, further cross-cultural research is needed to clarify its applicability and enable international comparisons.</p> Objective <p>This study aimed to validate the Croatian version of the EPOCH questionnaire and to introduce a revised Engagement subscale more closely aligned with Csikszentmihalyi’s flow theory, emphasizing autotelic, flow-facilitating activities.</p> Methods <p>Two adolescent samples participated. Sample 1 included 321 students (<i>M</i> = 16.27 years, <i>SD</i> = 1.64; 230 girls) who completed the EPOCH together with measures of life satisfaction, self-efficacy, depression, anxiety, and physical activity. These data were used to examine the factor structure via confirmatory factor analysis and to assess internal consistency, as well as convergent, discriminant, and concurrent validity. Sample 2 comprised 44 students (<i>M</i> = 16.80 years, <i>SD</i> = 0.39; 24 girls) who completed the EPOCH across three waves to assess test–retest reliability.</p> Results <p>CFA supported the expected five-factor structure, and the revised Engagement subscale demonstrated improved psychometric properties. All subscales showed satisfactory reliability and convergent and discriminant validity. Positive correlations emerged with life satisfaction and self-efficacy, and negative correlations with depression and anxiety. Measurement invariance across sex was confirmed, and the scale showed good temporal stability across retest intervals.</p> Conclusions <p>The Croatian version of the EPOCH demonstrated adequate psychometric properties. The revised Engagement subscale showed improved psychometric performance and clearer alignment with flow theory.</p>