The Seiyokan site is located in Kamakura, Japan, and has yielded 91 human skeletons belonging to the 14-15th centuries AD. The purposes of this study are to examine the human crania from the archeological site at Seiyokan, to analyze the presence, distribution, and variability of their weaponrelated traumas, and finally to better understand violence in medieval Japan from osteological evidence. The results demonstrate that the traumas on the crania have morphological features consistent with human-induced cut marks. The presence of cut marks on the Seiyokan crania, which strongly suggests the prevalence of violence in medieval Japan, is in accord with the historical background-that a militarily organized society was founded by force and that the rise to political power of the warrior class and the establishment of a military government resulted in continuous disturbances, armed conflicts, and violent death.