Background: Adolescents perceive both immediate and long-term benefits and losses related to internet gaming, affecting their risk of internet gaming disorder (IGD). These perceptions could also be shaped and reinforced by IGD, indicating potential bidirectionality. Objective: This study aimed to investigate the bidirectional relationships between perceived immediate and long-term benefits in 3 domains (mental health, social relationships, and personal achievement) and IGD, and between perceived immediate and long-term losses in 6 domains (mental health, sleep quality, academic performance, family relationships, social relationships, and personal achievement) and IGD. Methods: A 12-month 2-wave prospective longitudinal study was conducted among junior middle school students who had played internet games in the past 12 months in Guangzhou and Chengdu, China, with a baseline survey (T1, December 2018) and the other identical follow-up survey conducted 1 year later (T2, December 2019). The participating schools were conveniently selected; all Grade 7 and 8 students were invited to self-administer the questionnaires in a classroom setting without the presence of the schoolteachers. The final sample size was 1173 students (mean age 12.5, SD 0.6 y; male: 693/1173, 59.1%). IGD was assessed by using the 9-item IGD checklist. Results: Cross-lagged panel analysis (adjusting for background factors) showed (1) stronger perceived immediate benefits of mental health (=.08, 95% CI 0.01-0.15) and personal achievement (=.10, 95% CI 0.01-0.20) at T1 significantly predicted more IGD symptoms at T2; (2) more IGD symptoms at T1 significantly predicted stronger perceived immediate and long-term benefits of social relationships (immediate: =.09, 95% CI 0.03-0.15; long-term: =.11, 95% CI:0.05-0.17) and personal achievement (immediate: =.12, 95% CI 0.06-0.18; long-term: =.10, 95% CI 0.04-0.16) at T2; (3) more IGD symptoms at T1 significantly predicted stronger perceived immediate and future losses in mental health (immediate: =.09, 95% CI 0.03-0.15; long-term: =.08, 95% CI 0.02-0.14), sleep quality (immediate: =.10, 95% CI 0.04-0.16; long-term: =.13, 95% CI 0.07-0.19), academic performance (immediate: =.09, 95% CI 0.04-0.15; long-term: =.07, 95% CI 0.01-0.13), and family relationships (immediate: =.11, 95% CI 0.05-0.17; long-term: =.10, 95% CI 0.04-0.16) at T2, as well as perceived long-term losses in social relationships at T2 (=.08, 95% CI 0.02-0.14). Conclusions: This study was innovative in integrating time perspective into both perceived benefits and losses of internet gaming, a cognitive dimension previously overlooked in literature. The current findings advance the field by revealing the unidimensional predictive effects of IGD on perceived immediate and long-term benefits and losses, with 2 exceptions of perceived immediate and long-term benefits of mental health and personal achievement conversely predicting IGD. These results contribute to the development of effective interventions: the cognitive components should go beyond the general pros and cons of gaming and target the potential temporal bias gamers hold.