This study examined the prospective relationships between preschoolers’ body mass index (BMI) and cognitive development. BMI, cognitive (i.e., Brigance), sex, and age data were collected from seven cohorts of children attending Head Start from 2012 to 2018. Children (N = 324) with two years of complete data were included. After controlling for the first year cognitive development scores, age, gender, and the cohort, the BMI was predictive of lower cognitive development scores in year two (B = −0.06, β = −0.14, t = −3.19, p = 0.002). Female sex (B = 2.69, β = 0.10, t = 2.30, p = 0.022) and older age (B = 0.02, β = 0.15, t = 3.20, p ≤ 0.001) were also shown to be statistically significant predictors of improved year two cognitive scores. The initial BMI scores were associated with poorer one year cognitive development scores in this sample of preschool children. (author abstract)