Memory deficits are a common complaint of patients with posttraumatic stress disorder (PTSD). Despite vivid trauma‐related memory, previous studies report memory impairment for nontrauma‐related stimuli when compared to controls, specifically in associative memory (Guez et al., 2011). Healthy individuals show hemispheric memory asymmetry with left‐prefrontal lateralization of encoding and right‐prefrontal lateralization of episodic retrieval, suggesting a role for interhemispheric communication in memory‐related tasks (Gazzaniga, ; Ringo, Doty, Demeter, & Simard, ). Because brain magnetic resonance imaging (bMRI) studies in PTSD patients report volume changes in various regions, including white matter and corpus callosum (CC), we aimed to test the relationship between memory deficits and CC volume in PTSD patients. We probed for specific alterations in associative memory in PTSD and measured the volume of subportions within the CC employing bMRI. Our main finding was a reduction in CC white‐matter volume in PTSD patients, as compared to controls, t(35) = −2.7, p = .010, that was correlated with lower associative performance (r = .76, p = .003). We propose that CC volume reduction is a substrate for the associative memory deficits found in PTSD.