Objective: This study aimed to assess the status and characteristics of adverse events caused by treatments of coronavirus disease 2019 (COVID-19) in hospitalized patients in Korea. Methods: The medical records of patients hospitalized with COVID-19 at 30 medical institutions nationwide from January 1, 2020 to November 30, 2021 were retrospectively reviewed. Data of clinical characteristics; type of COVID-19 treatments; symptoms and severity of adverse events according to CTCAE classification; and prognosis of each patient were analyzed and presented descriptively. Results: Adverse events were observed in 853 of 5,740 (14.9%) hospitalized patients with COVID-19: 732 (85.8%), mild; 106 (12.4%), moderate; and 15 (1.8%), severe. Serious adverse events were observed in 70 (1.2%) patients, with 56 prolonged hospitalizations and four deaths. Adverse events were more frequently found in patients with a lower-than-normal body mass index or many concomitant medications. Moreover, 4,912 patients (85.6%) received multiple medications for treatment of COVID-19, wherein the drugs most mainly administered were steroids (66.4%), antibiotics (59.5%), and remdesivir (52.6%). Adverse events were relatively common in patients administered immunoglobulin, other antiviral drugs, and interleukin-6 inhibitors. Each patient with adverse events had an average of 3.8 symptoms. The most frequent symptoms were increased hyperbilirubinemia (n = 256), nausea (n = 216), and pruritis (n = 188). Conclusion: The incidence of adverse events in hospitalized patients with COVID-19 in Korea was approximately 14.9%. The type of treatment of COVID-19 might affect the incidence and prognosis of specific adverse events. Clinicians should consider the possible adverse effects of each medication before initiating treatment.