<p>Positive childhood experiences play a protective role in long-term emotional development and contribute to subjective happiness by fostering adaptive psychological processes. Although psychological flexibility has been identified as a potential mechanism underlying these associations, previous studies have relied predominantly on cross-sectional designs, limiting understanding of the longitudinal relationships among positive childhood experiences, psychological flexibility, and subjective happiness. This study longitudinally examined whether psychological flexibility mediates the relationship between positive childhood experiences and subjective happiness. The sample consisted of 376 Turkish university students (288 females, 76.6%) aged between 18 and 38 years (M = 20.37, SD = 1.75). Participants completed measures of positive childhood experiences, psychological inflexibility, and subjective happiness. A two-wave half-longitudinal mediation design was employed with a four-month interval. Half-longitudinal analysis revealed that positive childhood experiences at Time 1 predicted lower psychological inflexibility at Time 2 (β = −0.13, p &lt; .05), which in turn predicted higher subjective happiness at Time 2 (β = −0.13, p &lt; .05), indicating a partial longitudinal mediation effect. The findings suggest that early supportive environments enhance psychological flexibility, which subsequently contributes to greater subjective happiness. These results highlight the importance of fostering positive childhood experiences to promote well-being and suggest that interventions aimed at enhancing psychological flexibility may be particularly beneficial for emerging adults with fewer positive childhood experiences.</p>