Williams syndrome is a neurodevelopmental genetic disorder caused by a hemizygous deletion on chromosome 7q11.23, resulting in atypical brain structure and function, including abnormal morphology of the corpus callosum. An influence of handedness on the size of the corpus callosum has been observed in studies of typical individuals, but handedness has not been taken into account in studies of callosal morphology in Williams syndrome. We hypothesized that callosal area is smaller and the size of the splenium and isthmus is reduced in individuals with Williams syndrome compared to healthy controls, and examined age, sex, and handedness effects on corpus callosal area. Structural magnetic resonance imaging scans were obtained on 25 individuals with Williams syndrome (18 right-handed, 7 left-handed) and 25 matched controls. We found that callosal thickness was significantly reduced in the splenium of Williams syndrome individuals compared to controls. We also found novel evidence that the callosal area was smaller in left-handed participants with Williams syndrome than their right-handed counterparts, with opposite findings observed in the control group. This novel finding may be associated with LIM-kinase hemizygosity, a characteristic of Williams syndrome. The findings may have significant clinical implications in future explorations of the Williams syndrome cognitive phenotype.