Vaccination against COVID-19 has been promoted as a way to protect oneself and others from infection. To ensure the comprehensive acceptance of COVID-19 vaccines, the factors that affect COVID-19 vaccination hesitancy should be examined. This study aimed to identify the factors that affect COVID-19 vaccination hesitancy among Korean adults, focusing on COVID-19 knowledge and attitudes toward vaccination. This was a cross-sectional study conducted with 2286 Korean adults aged 20-64 years. Demographic characteristics, knowledge regarding COVID-19, and attitudes toward vaccination were assessed. A generalized linear model with Poisson regression was used to identify factors associated with COVID-19 vaccination hesitancy. Approximately 67% of the COVID-19 knowledge-related questions were correctly answered. A neutral attitude toward vaccination and relatively low vaccination hesitancy were found. COVID-19 vaccination hesitancy was affected by individuals' attitudes toward vaccination, as well as their gender and age, but not by their knowledge. Interventions or policies considering gender and age could be helpful in reducing COVID-19 vaccination hesitancy. Strategies to disseminate more accurate and novel information related to the COVID-19 vaccines should be implemented.