<p>This study explores the longitudinal relationship between psychological inflexibility and non-suicidal self-injury (NSSI) in adolescents, addressing a gap in the existing literature. A three-wave longitudinal design was adopted, with a random-intercept cross-lagged panel model (RI-CLPM) applied to differentiate between-person and within-person effects. The findings revealed a robust model fit (<i>χ</i>^<i>2</i>(43) = 280.733, <i>p</i> &lt; 0.001, CFI = 0.96, RMSEA = 0.04 (95%CI [0.04, 0.05])). At the between-person level, there was no significant association between psychological inflexibility and NSSI (<i>β</i> = 0.54, <i>SE</i> = 1.73, <i>p</i> = 0.75), whereas psychological inflexibility significantly predicted NSSI at the within-person level (<i>β</i> = 0.16, <i>SE</i> = 0.06, <i>p</i> = 0.01). Additionally, the study identified that external factors like location of residence and family economic status significantly influenced NSSI. These findings contribute to a deeper understanding of the role of psychological inflexibility in NSSI and highlight the importance of targeting this construct in therapeutic interventions for adolescents. Through a longitudinal approach, the study provides valuable insights into the dynamic mechanisms underlying NSSI behaviors, offering significant implications for clinical practice and future research.</p>