As the largest interhemispheric commissure in the brain, the corpus callosum is of particular interest in disorders that may preferentially affect white matter development such as Williams syndrome (WS). Individuals with WS possess a remarkable array of neurobehavioral peaks and valleys, including deficits in visuospatial ability, mathematics, and attention, but with relative preservation of language and affect. Our study measured the corpus callosum and its primary subdivisions using high-resolution MRI in 20 individuals with WS (13 females and seven males; mean age 28.5, SD 8.3 years; range 19 to 44 years) and 20 age- and sex-matched control participants (mean age 28.5, SD 8.2 years; range 19 to 48 years). Total midsagittal corpus callosum area was reduced (F=4.5, p=0.04, df=36) in the WS population. The area of the splenium (F=12.4, p=0.001, df=36) and isthmus (F=9.4, p=0.004, df=36) were disproportionately reduced in WS beyond the absolute reduction of the entire corpus callosum. These reductions are in concordance with other neuroanatomical findings of decreased parietooccipital volumes as well as the observed visuospatial problems associated with WS.