Neurofibromatosis type 1 (NF1) is associated with cognitive dysfunction and structural brain abnormalities such as an enlarged corpus callosum. This study aimed to determine the relationship between corpus callosum morphology and cognitive function in children with neurofibromatosis type 1 using quantitative neuroanatomic imaging techniques. Children with neurofibromatosis type 1 (n = 46) demonstrated a significantly larger total corpus callosum and corpus callosum index compared with control participants (n = 30). A larger corpus callosum index in children with neurofibromatosis type 1 was associated with significantly lower IQ, reduced abstract concept formation, reduced verbal memory, and diminished academic ability, specifically reading and math. Our results suggest an enlarged corpus callosum in children with neurofibromatosis type 1 is associated with cognitive impairment and may provide an early structural marker for the children at risk of cognitive difficulties. Cognitive deficits associated with structural brain abnormalities in neurofibromatosis type 1 are unlikely to be reversible and so may not respond to proposed pharmacological therapies for neurofibromatosis type 1-related cognitive impairments.