Abstract Background Obesity is highly stigmatized, with negative obesity-related stereotypes widespread across society. Internalized weight stigma (IWS) is linked to negative outcomes including poor mental health and disordered eating. Previous evidence examining population groups at higher risk of experiencing IWS comes from small, nonrepresentative samples. Here, we re-assess previously reported associations of IWS with social factors in a large general population birth cohort study for the first time. Methods In the Avon Longitudinal Study of Parents and Children (ALSPAC), we explored differences in IWS at age 31 years by sex, ethnicity, socioeconomic factors, and sexual orientation using multivariable regression. Using data collected over a 32-year period, we explore the impact of family, peer, and wider social influences in adolescence, and bullying from mid-childhood to adulthood, whilst minimising risk of recall bias. Results In models adjusted for potential confounders and BMI in childhood, adolescence, and adulthood (N = 4060), IWS was higher for females (0.56 S.D. higher, CI:0.50-0.61), sexual minorities (0.17 S.D. higher, CI:0.09-0.24), and less socioeconomically advantaged individuals (0.16 S.D. higher (CI:0.08-0.24) for those whose mothers had minimum or no qualifications, compared to a university degree). The social environment during adolescence and young adulthood was important: IWS was higher for people who at age 13 years felt pressure to lose weight from family (0.13 S.D., CI:0.03-0.23), and the media (S.D. 0.17, CI:0.10-0.25), or had experienced bullying (0.25 S.D., CI:0.17-0.33 for bullying at age 23 years). Conclusions Internalized weight stigma differs substantially between demographic groups, and this is not explained by differences in BMI. The family environment during adolescence, bullying, and pressure to lose weight from the media may have long-lasting impacts on adult IWS. These may be fruitful avenues for intervention to reduce IWS and its consequences. Key messages • Females, sexual minorities, and socioeconomically disadvantaged adults are at greater risk of IWS irrespective of BMI. • To reduce IWS, interventions should improve the family and social environment.