The Late Iron Age to early historic period site of Non Ban Jak (ca. fourth to eighth centuries a.d.), in the Upper Mun River Valley, northeast Thailand, provides one of the largest and best preserved skeletal samples from an important period of change in social and technological complexity in prehistoric Southeast Asia. This study assessed the patterning of individual-level trauma prevalence and long bone fracture rates in this community to determine the level of risk of injury and the possible relationship with cultural change. One quarter (25.5%) of adults experienced antemortem or perimortem trauma. This level of trauma is high when compared to other Iron Age sites in the Southeast Asian region. Injury patterning suggests the majority of antemortem trauma is the result of accidental injury through occupational hazards and lifestyle activities. However, cranial injuries such as a depressed fracture, facial fractures, and perimortem sharp force trauma, as well as a possible parry injury and multiple trauma observed in some individuals, are indicative that at least a portion of the population had been engaged in interpersonal violence. This study emphasizes local variation. Non Ban Jak is one of only a few excavated sites that contributes to an understanding of the transition from the late Iron Age to the early protohistoric period in Thailand as well as the wider region in Southeast Asia.