<p>Existing evidence has suggested a positive association between school connectedness and bystanders’ defending behavior in bullying contexts. However, the directionality and underlying mechanisms of this effect remain unclear. The present study examined the bidirectional relationships between school connectedness and bystanders’ defending behavior in bullying, as well as investigated personal mastery as a mediator in these relationships. A total of 803 elementary school students (girls = 46.33%; <i>M</i>_age = 10.12, <i>SD</i>_age = 1.16) completed three waves of assessments, with six-month intervals between waves. This study combined the traditional cross-lagged panel model (CLPM) with the random-intercept cross-lagged panel model (RI-CLPM) to disentangle between-person and within-person effects. CLPM results showed a bidirectional association between school connectedness and personal mastery. The RI-CLPM results indicated that there was a bidirectional positive relation between bystanders’ defending behavior in bullying and personal mastery, and that personal mastery mediated the positive feedback cycle from defending behavior to itself. Both CLPM and RI-CLPM showed that defending behavior increased school connectedness through improved personal mastery. Meanwhile, the multi-group analyses of the RI-CLPM revealed no significant moderating effects of gender or socioeconomic status (SES). These findings underscore the importance of personal mastery for healthy development and highlight its pivotal role in bystanders’ defending behavior in bullying, offering insights for future anti-bullying interventions.</p>