Aims: Despite the high prevalence of both depression and smartphone addiction among Chinese adolescents, research examining their bidirectional relationship has been limited. Moreover, longitudinal research regarding the influence of maladaptive metacognitive beliefs on smartphone addiction is scarce. This 6-month longitudinal study aimed to address these research gaps by testing a cross-lagged panel model of maladaptive metacognition, depression, and smartphone addiction. Methods: Four hundred and fifty-nine Chinese high school students voluntarily completed an anonymous questionnaire at baseline, and 313 of them (36.1% male; age = 14 to 18; M = 16.88, SD = 0.62) completed the same questionnaire at follow-up. Results: Positive correlations were shown among depression, smartphone addiction, and maladaptive metacognition at both waves (r = 0.16 to 0.57, p <.01). The results of the cross-lagged analysis demonstrated only the prospective effect of depression on smartphone addiction (β = 0.18, p <.001) but not vice versa. Moreover, maladaptive metacognition assessed at baseline significantly predicted subsequent depression (β = 0.14, p <.01) but not smartphone addiction (p>.05). Additional path analysis showed a significant indirect effect of baseline maladaptive metacognition (0.099 [95% CI = 0.042, 0.183]) on subsequent smartphone addiction, via depression. Conclusions: Findings of this study showed that the relationship between depression and smartphone addiction was unidirectional rather than bidirectional among Chinese adolescents. Specifically, depression predicted smartphone addiction, while maladaptive metacognition predicted depression. Depression also mediated the relationship between maladaptive metacognition and smartphone addiction. The findings suggest that incorporating metacognition and depression interventions into smartphone addiction prevention designs for adolescents may be beneficial.