Background: Children are a vulnerable population in terms of the impact of COVID-19 on their psychological well-being. When restricted to their homes, children are susceptible to problematic Internet gaming (PG). Primary school boys are particularly at risk of PG, which may lead to negative psychological effects, such as distress. Emerging research has identified perceived weight stigma (PWS) as a variable closely associated with both PG and psychological distress, particularly during the COVID-19 pandemic. The purpose of this study was to evaluate the trajectory of psychological distress among this vulnerable population from a longitudinal perspective, evaluating the role of PG and PWS. Methods: Self-report measures were used to assess psychological distress, PG, and PWS among primary school boys (grades 4 to grade 6; N = 283). Data were collected across three waves: before the pandemic, during school closure, and following the lifting of restrictions. Results: The trajectory of psychological distress among primary school boys was concave, indicating their mental health was negatively impacted during home restriction but recovered after the lockdown ended (linear change = 0.98, p 0.01; quadratic change = –0.19, p 0.01). PG was a significant covariate in terms of the trajectory of psychological distress (b = 0.02, p 0.01). Moreover, baseline values for PWS were shown to have a negative direct effect on mental health before the pandemic (b = 0.05, p 0.01), and moderated the time factor for boys’ psychological distress over time (b of PWS \times linear change = 0.04, p = 0.006; b of PWS \times Quadratic change was negative at –0.01, p = 0.002). Conclusions: Although mental health gradually improved as home restrictions subsided, future studies are required to address changes in mental health upon return to school for students reporting higher levels of weight stigma.