Although the negative relationship between BMI and academic achievement (AA) is well documented, no prior studies have investigated the potential bi-directional relationship between BMI and AA in childhood. We investigated the longitudinal relationships between child BMI and AA across different school subjects (reading, math and science) and sexes. To do so, we employed the Early Childhood Longitudinal Study kindergarten cohort (2011), which is a nationally representative sample of American children who entered kindergarten in 2010–2011. We utilised the kindergarten–fifth grade longitudinal sample (n 17 480) and applied cross-lagged panel models with fixed effects to address unobserved heterogeneity. Our results showed significant but small reciprocal relationships between BMI and math/science achievement for girls (n 8540) (year-to-year effect sizes ranged from –0·01 to –0·04), but not for reading. In contrast, we did not find any evidence of reciprocal relationships between BMI and AA for boys (n 8940). Our results reveal that early weight status and academic performance may be jointly responsible for a vicious cycle of poor AA and unhealthy weight. Breaking the cycle from AA may complement existing obesity prevention strategies, particularly for girls in the science, technology, engineering and mathematics field.