Abstract Background Weight stigma is an issue often studied in Western countries; however, such information is scarce in Asian studies. Methods This study aimed to examine the role of internalized weight stigma as a mediator in the relationship between perceived weight stigma and changes in body mass index (BMI). The data were collected through a longitudinal online survey with two phases (n = 974; Phase 1: August and September 2021; Phase 2: November and December 2021). The Perceived Weight Stigma Scale (PWSS), Weight Self-Stigma Questionnaire (WSSQ), and Depression, Anxiety, Stress Scale − 21 (DASS-21) were administered to assess perceived weight stigma, internalized weight stigma, and psychological distress. Hierarchical regressions were used to examine the proposed model, and Hayes’ Process Macro was used to test a mediation model. Results The changes in perceived weight stigma and internalized weight stigma were significantly and positively associated with changes in BMI. There were significant and positive associations between perceived weight stigma, internalized weight stigma and psychological distress over time. Change in internalized weight stigma was found to be a significant mediator in the association of change in perceived weight stigma with change in BMI for the entire sample (unstandardized coefficient = 0.04; 95% CI = 0.02, 0.06), female subgroup (unstandardized coefficient = 0.05; 95% CI = 0.02, 0.08), and male subgroup (unstandardized coefficient = 0.03; 95% CI = 0.01, 0.06). Change in perceived weight stigma also had significant effects on change in BMI for the entire sample and the female subgroup, but not for the male subgroup. Conclusion Because perceived weight stigma may significantly impact changes in BMI through internalized weight stigma, treatment strategies to ameliorate self-stigma may enhance the results of weight-reduction programs. Such treatment strategies should be considered for inclusion in weight-loss interventions.