Objective The injury-related disease burden in South Korea is relatively high compared to that in other nations. This study was conducted to identify the scale and causes of injury by severity, using an injury pyramid and analyzing the sex and age-dependent differences and the basic demographic characteristics. Methods Unintentional injury was estimated for each group after classifying injury-related deaths, hospitalizations, and outpatient department (OPD) visits based on their severity. The injury pyramid was calculated by classifying the injury outcomes into deaths, hospitalizations, and outpatient visits. Results The incidence of unintentional injury included 31.74 deaths, 1,715.27 hospitalizations, and 7,317.55 OPD visits per 100,000 persons. The incidence of injury was higher among males than that among females. There were more hospitalizations and OPD visits than deaths among females. The mortality and hospitalization rates due to injury per 100,000 persons were the highest among those ≥75 years old, and the OPD visit rate was the highest among preschool children aged 0 to 6 years. The injury mechanisms that caused most deaths and hospitalizations in South Korea were injuries related to traffic, falls, poisoning, and burns/fires. Conclusion The scale of unintentional injury varies based on sex, age, and injury mechanism; additionally there are differences in the incidence of injuries between males and females depending on their age and injury mechanism. The high incidence of injury in elderly people could be a factor that increases the burden of injury in South Korea; hence, it is necessary to develop an injury prevention program that targets the elderly.