Reductions in the size of the anterior callosum have been described for both first-episode schizophrenia-spectrum psychosis and established schizophrenia, but have not been examined in individuals at ultra-high risk for psychosis (UHR). We compared 100 UHR individuals (27 of whom later developed psychosis) with 38 age-matched control subjects on measures of size and shape of the corpus callosum to determine if changes previously demonstrated in first-episode and established schizophrenia are present in the pre-psychotic phase. Each individual's callosum was extracted from the mid-sagittal slice from T1-weighted magnetic resonance images, and total area, length and curvature of the callosum was compared using one-way ANOVA, and 39 regional thicknesses via a non-parametric permutation method to account for non-independence of adjacent measures. Total area, length and curvature did not differ between the groups. Compared to both the UHR-NP group and controls, the UHR-P group showed significant regional reductions in the region of the anterior genu of the callosum. The UHR-NP group did not differ from controls. Positive and negative symptoms did not affect regional thickness in either of the patient groups. Cox regression showed that mean anterior genu thickness was highly predictive of a transition to psychosis. Reductions in the thickness of the anterior callosum differentiate between high-risk individuals who transition to psychosis and those who do not, and is highly predictive of transition. These changes may reflect primary pathology of orbitofrontal and medial frontal cortex, or deficits in anterior interhemispheric myelination.