Objective: To test the hypothesis that obesity is associated with impaired cognitive outcomes in the preschool years. Methods: Associations were examined between weight status at age 3-5 years and cognitive performance at age 5 years. Cognitive outcome measures were tests of pattern construction (visuospatial skills), naming vocabulary (expressive language skills), and picture similarity (reasoning skills). The sample was the UK Millennium Cohort Study (n = 12,349 participants). Results: Boys with obesity at 3 years had significantly lower performance in pattern construction at age 5 years compared to those of a healthy weight, even after controlling for confounders (beta = -0.029, P = 0.03). Controlling for confounders, boys who developed obesity between the ages of 3 and 5 years had lower scores in pattern construction (beta = -0.03, P = 0.03). ‘Growing out’ of obesity had a positive association with picture similarity performance in girls (beta = 0.03, P = 0.04). Conclusions: Obesity in the pre-school years was associated with poorer outcomes for some cognitive measures in this study. Stronger relationships between obesity and cognition or educational attainment may emerge later in childhood.