Objectives This study aimed to explore how the coronavirus disease-2019 (COVID-19) pandemic affected the mental health of university hospital nurses with a focus on their resilience, stress, depression, anxiety, and sleep. Methods A total of 220 nurses working at a university hospital in South Korea during the COVID- 19 pandemic participated in this study. The collected data were subjected to analysis based on the percentage, mean, standard deviation, Kruskall-Wallis test, Mann-Whitney test, and Pearson correlation coefficient using the IBM SPSS Statistics 25 program. Results The range of the Brief Resilience Scale was 1.3 to 5.0 (3.2±0.6), Perceived Stress Scale was 4 to 34 (18.6±5.3), Pittsburgh Sleep Quality Index was 2 to 17 (7.4±3.1), Patient Health Questionnaire was 0 to 18 (5.0±4.0), and General Anxiety Disorder-7 was 0 to 16 (3.4±3.4). The range of the difference in job stress before and after the COVID-19 pandemic outbreak was -5 to +7 (2.2±1.9). There was a negative correlation between the nurses’ resilience and stress, depression, anxiety, and sleep. Conclusion Most of the nurses experienced high levels of stress, depression, anxiety, and poor sleep quality during the COVID-19 pandemic. The mental health of nurses should be carefully monitored with interest, and various compensatory systems should be developed to help them cope with the situation.