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• Gender inequalities in youth physical activity are not explained by country/region-level school uniform policies. Findings remain consistent when analyses are restricted to device-measured data. • However, among primary school-aged children, greater gender inequalities in physical activity are observed in countries/regions where school uniforms are common. • The association between school uniform policies and gender inequalities in youth physical activity is largest in high-income countries/regions. • Compliance with physical activity guidelines is lower in countries/regions where school uniforms are common compared to countries/regions where uniforms are less commonly worn. It is aimed to assess whether school uniforms are associated with population-level gender inequalities in physical activity, and whether associations differ by school level, country/region income, and assessment method. An ecological study design was employed. We collected data about global uniform practices using an online survey. We searched for country/region-level estimates of school-aged youth meeting physical activity guidelines from international surveillance studies. Study selection was conducted in duplicate using a systematic process, and a random sample of all data was checked to ensure extraction and pooling processes were accurate. We calculated absolute and relative gender inequalities in physical activity for each country. Linear regression examined associations between country/region-level uniform practices (binary yes/no exposure variable) and country/region-level gender inequalities in physical activity guideline compliance (absolute and relative inequalities). We investigated moderation by school level, stratified analyses by income group, and repeated primary analyses using device-measured data. Pooling data from 135 countries/regions (n = 1,089,852), we found no association between population-level uniform practices and gender inequalities in physical activity across all ages (absolute: β = –0.2; 95% confidence interval (95%CI): –1.7 to 1.3, p = 0.74; relative: β = 0.1; 95%CI: –0.1 to 0.2, p = 0.51). Subgroup analysis suggested a positive association in primary school settings (absolute: β = 4.3; 95%CI: –0.0 to 8.6, p = 0.05). Among high-income countries, absolute inequalities were significantly greater in countries/regions with uniform practices (N = 37) compared to those without (N = 48) (9.1 (SD = 3.6) vs. 7.8 percentage points (SD = 4.3)). Repeating analyses using device-measured data (n = 32,130; N = 24) did not alter our primary finding. From initial descriptive statistics, we found that in countries/regions where a majority of schools (>50%) reportedly use uniforms, there was lower compliance with physical activity guidelines among all genders (median: 16.0%, interquartile range: 13.2%–19.9%, N = 103) compared to generally non-uniform countries/regions (median: 19.5%, interquartile range: 16.4%–23.5%, N = 32) (z = 3.04, p = 0.002). (N = countries, regions and studies represented; n = sample size or participants included). School uniforms are associated with greater gender inequalities in physical activity in primary school settings and in high-income countries. Our population-level findings warrant testing using individual-level data across contexts. [Display omitted] [ABSTRACT FROM AUTHOR]