This is the first detailed study of the relation between cesarean birth and child cognitive development. We measure differences in child cognitive performance at 4 to 9 years of age between cesarean-born and vaginally-born children (n = 3,666) participating in the Longitudinal Study of Australian Children (LSAC). LSAC is a nationally representative birth cohort surveyed biennially. Using multivariate regression, we control for a large range of confounders related to perinatal risk factors and the socio-economic advantage associated with cesarean-born children. Across several measures, we find that cesarean-born children perform significantly below vaginally-born children, by up to a tenth of a standard deviation in national numeracy test scores at age 8-9. Estimates from a low-risk sub-sample and lower-bound analysis suggest that the relation is not spuriously related to unobserved confounding. Lower rates of breastfeeding and adverse child and maternal health outcomes that are associated with cesarean birth are found to explain less than a third of the cognitive gap, which points to the importance of other mechanisms such as disturbed gut microbiota. The findings underline the need for a precautionary approach in responding to requests for a planned cesarean when there are no apparent elevated risks from vaginal birth.