<p>Children are highly vulnerable to air pollution, yet most studies rely on single-pollutant approaches. This study examined associations between individual and joint exposure to multiple air pollutants and cognitive outcomes in preschoolers, with attention to potential sex differences.&#xa0;Traffic-related pollutants (PM_2.5, PM₁₀, PM_coarse, PM_{2.5absorbance}, NO₂, NO_x, O₃) were estimated at schools from the cross-sectional study among 286 children. Cognitive function was assessed with the Wechsler Preschool and Primary Scale of Intelligence–IV. Linear and weighted quantile sum (WQS) regression were conducted, adjusting for relevant child and family covariates.&#xa0;PM_coarse (<i>β</i> = −2.71, 95% CI −4.23, -1.20) and PM₁₀ (<i>β</i> = −2.39, 95% CI −4.09, −0.70) were inversely associated with the working memory index (WMI), with stronger associations observed in boys (PM_coarse: <i>β</i> = −3.32, 95% CI −5.77, −0.88; PM₁₀: <i>β</i> = −3.79, 95% CI −6.50, −1.09). WQS analysis indicated a negative association between the pollutant mixture and WMI (estimate = − 3.60, 95% CI −5.92, −1.28), with PMcoarse contributing most to the index. Sex-stratified WQS analyses suggested associations between air pollution mixture and lower verbal comprehension in girls and lower WMI in boys.</p><p><i>Conclusion</i>:&#xa0;School-age exposure to ambient air pollution mixtures was associated with differences in cognitive performance among preschool children, with patterns varying by sex. These findings highlight the importance of considering pollutant mixtures and potential sex differences when evaluating associations between air pollution and early childhood cognition.</p><p><i>Clinical trial registration:</i> EUCTR-2012-005480-28 (<a href="http://www.clinicaltrialsregister.eu">www.clinicaltrialsregister.eu</a>).&#xa0;NCT03196882 (<a href="http://www.clinicaltrials.gov">www.clinicaltrials.gov</a>).<Table Float="No" ID="Taba"> <tgroup cols="2"> <colspec align="left" colname="c1" colnum="1" /> <colspec align="left" colname="c2" colnum="2" /> <tbody> <row> <entry align="left" nameend="c2" namest="c1"> <p><b>What is Known:</b></p> <p><i>• Ambient air pollution is linked to impaired neurodevelopment in children, but evidence using multi-pollutant mixture models in preschoolers remains limited.</i></p> </entry> </row> <row> <entry align="left" nameend="c2" namest="c1"> <p><b>What is New:</b></p> <p><i>• Air pollutant mixture was negatively associated with working memory index in preschoolers.</i></p> <p><i>• Sex-stratified analyses suggested potential associations with lower verbal comprehension in girls and lower working memory in boys.</i></p> </entry> </row> </tbody> </tgroup> </Table></p>