BackgroundBody dissatisfaction is associated with poor psychological and physical health, particularly among young people. However, limited data exist on body size perceptions across countries and factors associated with dissatisfaction. ObjectiveThis study examined dissatisfaction prevalence and associations with sociodemographics and social media use among youth in 6 countries. DesignRepeat cross-sectional national online surveys were conducted as part of the 2019 and 2020 International Food Policy Study Youth Survey. ParticipantsThe sample included 21 277 youth aged 10 to 17 years from Australia, Canada, Chile, Mexico, the United Kingdom, and the United States. Youth were recruited to complete the online survey through parents/guardians enrolled in the Nielsen Consumer Insights Global Panel and their partners’ panels. Main outcome measuresFigural drawing scales assessed self-perceived and ideal body images, with differences between scales representing body dissatisfaction. Statistical analyses performedMultinomial logistic regression models examined differences in body dissatisfaction by country, and associations with sociodemographics and either social media screen time or platforms used, including 2-way interactions with country. ResultsOverall, approximately 45% of youth reported the same perceived and ideal body sizes, whereas 35% were “larger than ideal” (from 33% in Canada and Australia to 42% in Chile) and 20% were “thinner than ideal” (from 15% in Chile to 22% in Mexico). Greater social media screen time was associated with a higher likelihood of moderate-severe dissatisfaction for being “thinner than ideal” and at least mild dissatisfaction for being “larger than ideal” (P < .003 for all contrasts), with greater dissatisfaction among users of YouTube and Snapchat than nonusers (P ≤ .005 for both contrasts). Modest differences in body dissatisfaction between countries were observed for age, ethnicity, body mass index, and weight-based teasing. ConclusionsBody dissatisfaction is prevalent among youth across diverse countries. These findings highlight the need to promote healthy body image in youth, particularly among social media users.