BACKGROUND: Internet gaming disorder (IGD) is a global public health concern for adolescents due to its potential severe negative consequences. Professional help-seeking is important for early screening, diagnosis, and treatment of IGD. However, research on the factors associated with professional help-seeking for IGD as well as relevant mediation mechanisms among adolescents is limited. OBJECTIVE: Based on the stress coping theory, the conservation of resource theory, and behavioral change theories, this study investigated the prevalence and factors influencing the behavioral intention of professional help-seeking for internet gaming disorder (BI-PHSIGD). The research also explored the underlying mechanisms, including psychosocial resources like resilience and social support, perceived resource loss due to reduced gaming time, and self-efficacy, in professional help-seeking among adolescent internet gamers. METHODS: A cross-sectional survey was conducted among secondary school students who were internet gamers in 2 Chinese cities from October 2019 to January 2020. Data from the full sample (N=1526) and a subsample of 256 IGD cases (according to the 9-item DSM-5 [Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition] IGD Checklist) were analyzed. Multivariate logistic regression analysis was conducted to examine the factors of BI-PHSIGD, while structural equation modeling was performed to test the proposed mediation mechanisms. RESULTS: The prevalence of BI-PHSIGD was 54.3% (829/1526) in the full sample and 40.6% (104/256) in the IGD subsample (vs 708/1239, 57.1% among non-IGD cases). In the full sample, psychosocial resources of resilience (adjusted odds ratio [aOR] 1.03, 95% CI 1.02-1.05) and social support (aOR 1.03, 95% CI 1.02-1.04) as well as self-efficacy in professional help-seeking (aOR 1.64, 95% CI 1.49-1.81) were positively associated with BI-PHSIGD, while perceived resource loss due to reduced gaming time was negatively associated with BI-PHSIGD (aOR 0.97, 95% CI 0.96-0.98); the positive association between psychosocial resources and BI-PHSIGD was fully mediated via 2 single-mediator indirect paths (via self-efficacy in professional help-seeking alone: effect size=53.4%; indirect effect/total effect=0.10/0.19 and via perceived resource loss due to reduced gaming time alone: effect size=17.8%; indirect effect/total effect=0.03/0.19) and one 2-mediator serial indirect path (first via perceived resource loss due to reduced gaming time then via self-efficacy in professional help-seeking: effect size=4.7%; indirect effect/total effect=0.009/0.19). In the IGD subgroup, a full mediation via self-efficacy in professional help-seeking alone but not the other 2 indirect paths was statistically significant. CONCLUSIONS: Many adolescent internet gamers, especially those with IGD, were unwilling to seek professional help; as a result, early treatment is often difficult to achieve. To increase BI-PHSIGD, enhancing psychosocial resources such as resilience and social support, perceived resource loss due to reduced gaming time, and self-efficacy in professional help-seeking may be effective. Future longitudinal and intervention studies are needed to confirm and extend the findings.