Purpose: This study investigates the characteristics and prognosis of acute poisoning patients visiting nationwide emergency departments before and after the Coronavirus disease 2019 (COVID-19) pandemic. Data were obtained from the National Emergency Department Information System (NEDIS). Methods: This nationwide retrospective observational study included acute poisoning patients who visited the emergency departments between February 1 to December 31, 2020, which has been determined as the pandemic period. The same periods in 2018 and 2019 were designated as the control periods. The primary outcome assessed was the length of stay in emergency departments (EDLOS). The secondary outcomes examined were intensive care unit admission rate and in-hospital mortality rate before and after the pandemic. A subgroup analysis was performed for inpatients and intentional poisoning patients. Results: A total of 163,560 patients were included in the study. During the pandemic, the proportion of women increased from 50.0% in 2018 and 50.3% in 2019 to 52.5% in 2020. Patients aged 20-29 years increased from 13.4% in 2018 and 13.9% in 2019 to 16.6% in 2020. A rise in cases of intentional poisoning was also noted - from 33.9% in 2018 and 34.0% in 2019 to 38.4% in 2020. Evaluating the hospitalized poisoned patients revealed that the EDLOS increased from 3.8 hours in 2018 and 3.7 hours in 2019 to 4.2 hours in 2020. ICU admissions were also markedly increased (2018, 48.2%; 2019, 51.8%; 2020, 53.2%) among hospitalized patients. Conclusion: The COVID-19 pandemic has changed the epidemiology, clinical characteristics, and prognosis of acute poisoning patients visiting nationwide emergency departments in Korea. The proportion of young adults, women, and intentional poisoning patients has increased after the COVID-19 pandemic. Prolonged length of stay at the emergency department and an increased rate of intensive care unit admissions were determined in hospitalized acute poisoning patients.