["Journal of Child Psychology and Psychiatry, EarlyView. ", "\n\nBackground\nEvidence on the association between loneliness and psychotic experiences in adolescents remains limited. Moreover, loneliness has typically been assessed at a single time point, which fails to capture its dynamic nature. We hypothesized that persistent loneliness, assessed across repeated measures, would be associated with psychotic experiences and other mental health problems.\n\n\nMethods\nUsing longitudinal data from 3,171 participants in the Tokyo Teen Cohort, we applied the g‐formula. We analyzed how loneliness patterns at ages 12 and 14 were associated with psychotic experiences, depression, anxiety, and diminished well‐being at age 16, accounting for time‐fixed and time‐varying confounders. Missing data were handled using multiple imputation by chained equations.\n\n\nResults\nPersistent loneliness was associated with increased risk and greater severity of psychotic experiences (RD 7.1%, 95% CI: 0.8–14.3; RR 2.44, 95% CI: 1.16–4.11; β 0.28, 95% CI: 0.10–0.48). Incident loneliness at age 14 showed similar associations. No association was found for adolescents whose loneliness had remitted by age 14 (RD −1.3%, 95% CI: −3.6 to 1.2; RR 0.73, 95% CI: 0.31–1.26; β 0.01, 95% CI: −0.04 to 0.08). Sensitivity analyses using marginal structural models yielded results that were largely unchanged. Findings were generally similar for other mental health problems. Associations were consistent across genders, although the association with well‐being appeared particularly important for girls.\n\n\nConclusions\nThe dynamics of loneliness are associated with a wide range of mental health problems in adolescents. The risk may not be permanent and could be mitigated if loneliness remits. Further research examining interventions that target loneliness is warranted.\n\n"]