Purpose: Suicide ranks among the top causes of death among youth in South Korea. This study aimed to identify the characteristics of suicidal individuals treated at emergency departments between 2011 and 2020. Methods: A retrospective analysis was conducted using data from January 2011 to December 2020 in the Injury Surveillance Cohort, a prospective registry. Patients’ sex, age, mortality, methods of self-harm, and previous suicide attempts were analyzed. The methods of self-harm were categorized into falls, asphyxiation, blunt injuries, penetrating injuries, poisoning, and others. Sub-groups with and without poisoning were compared. Results: The proportion of self-harm/suicide attempts increased from 2.3% (2011) to 5.0% (2020). The mortality rate decreased from 10.8% (2011) to 6.3% (2020). Poisoning was the most common method (61.7%). Mortality rates ranged from 42.0% for asphyxiation to 0.2% for blunt injuries. Individuals in their 20s showed a marked increase in suicide/self-harm attempts, especially in the last three years. A large proportion of decedents in their 70s or older (52.6%) used poisoning as a method of suicide. The percentage of individuals with two or more previous attempts rose from 7.1% (2011) to 19.7% (2020). The death rates by poisoning decreased from 7.7% (2011) to 2.5% (2020). Conclusion Our findings underscore the urgent need for targeted interventions and suicide prevention policies. Managing and reducing suicide and self-harm in emergency settings will require a focus on poisoning, the 10–29 age group, and the elderly. This paper will be valuable for future policies aiming to reduce the societal burden of suicide and self-harm.